Huge refactorig for consistent col/row numbering (#356)

* Huge refactorig for consistent col/row numbering

Started from simply changing ToALphaString()/TitleToNumber() logic and related fixes.
But have to go deeper, do fixes, after do related fixes and again and again.

Major improvements:

1. Tests made stronger again (But still be weak).
2. "Empty" returns for incorrect input replaces with panic.
3. Check for correct col/row/cell naming & addressing by default.
4. Removed huge amount of duplicated code.
5. Removed ToALphaString(), TitleToNumber() and it helpers functions at all,
and replaced with SplitCellName(), JoinCellName(), ColumnNameToNumber(), ColumnNumberToName(), CellNameToCoordinates(), CoordinatesToCellName().
6. Minor fixes for internal variable naming for code readability (ex. col, row for input params, colIdx, rowIdx for slice indexes etc).

* Formatting fixes

1 files changed, 21 insertions, 0 deletions

diff --git a/errors_test.go b/errors_test.go
new file mode 100644
index 0000000..89d241c
--- /dev/null
+++ b/errors_test.go
@@ -0,0 +1,21 @@
+package excelize
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewInvalidColNameError(t *testing.T) {
+	assert.EqualError(t, newInvalidColumnNameError("A"), "invalid column name \"A\"")
+	assert.EqualError(t, newInvalidColumnNameError(""), "invalid column name \"\"")
+}
+
+func TestNewInvalidRowNumberError(t *testing.T) {
+	assert.EqualError(t, newInvalidRowNumberError(0), "invalid row number 0")
+}
+
+func TestNewInvalidCellNameError(t *testing.T) {
+	assert.EqualError(t, newInvalidCellNameError("A"), "invalid cell name \"A\"")
+	assert.EqualError(t, newInvalidCellNameError(""), "invalid cell name \"\"")
+}