This closes #1096, memory usage optimization and another 4 changes

- Unzip shared string table to system temporary file when large inner XML, reduce memory usage about 70%
- Remove unnecessary exported variable `XMLHeader`, we can using `encoding/xml` package's `xml.Header` instead of it
- Using constant instead of inline text for default XML path
- Rename exported option field `WorksheetUnzipMemLimit` to `UnzipXMLSizeLimit`
- Unit test and documentation updated

1 files changed, 14 insertions, 15 deletions

diff --git a/xmlDrawing.go b/xmlDrawing.go
index 1690554..4ae6a29 100644
--- a/xmlDrawing.go
+++ b/xmlDrawing.go
@@ -37,21 +37,20 @@ var (
 
 // Source relationship and namespace.
 const (
-	SourceRelationshipOfficeDocument = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument"
-	SourceRelationshipChart          = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/chart"
-	SourceRelationshipComments       = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/comments"
-	SourceRelationshipImage          = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/image"
-	SourceRelationshipTable          = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/table"
-	SourceRelationshipDrawingML      = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/drawing"
-	SourceRelationshipDrawingVML     = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing"
-	SourceRelationshipHyperLink      = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/hyperlink"
-	SourceRelationshipWorkSheet      = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet"
-	SourceRelationshipChartsheet     = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/chartsheet"
-	SourceRelationshipDialogsheet    = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/dialogsheet"
-	SourceRelationshipPivotTable     = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/pivotTable"
-	SourceRelationshipPivotCache     = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/pivotCacheDefinition"
-	SourceRelationshipSharedStrings  = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings"
-
+	SourceRelationshipOfficeDocument             = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument"
+	SourceRelationshipChart                      = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/chart"
+	SourceRelationshipComments                   = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/comments"
+	SourceRelationshipImage                      = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/image"
+	SourceRelationshipTable                      = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/table"
+	SourceRelationshipDrawingML                  = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/drawing"
+	SourceRelationshipDrawingVML                 = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing"
+	SourceRelationshipHyperLink                  = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/hyperlink"
+	SourceRelationshipWorkSheet                  = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet"
+	SourceRelationshipChartsheet                 = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/chartsheet"
+	SourceRelationshipDialogsheet                = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/dialogsheet"
+	SourceRelationshipPivotTable                 = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/pivotTable"
+	SourceRelationshipPivotCache                 = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/pivotCacheDefinition"
+	SourceRelationshipSharedStrings              = "http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings"
 	SourceRelationshipVBAProject                 = "http://schemas.microsoft.com/office/2006/relationships/vbaProject"
 	NameSpaceXML                                 = "http://www.w3.org/XML/1998/namespace"
 	NameSpaceXMLSchemaInstance                   = "http://www.w3.org/2001/XMLSchema-instance"