Fixed #462 Handle multi row inline strings

Fix potential memory leak

The inline string struct is actually the same
as the shared strings struct, reuse it.

Note that Go version 1.10 is required.

Fixes #462

Get sheet names based on index

Fix potential memory leak where zw is not Close() when an error occurs.

SheetID only seems to indicate the file name for the sheet.
Check the sheets list based on index instead. Reordering sheets
in Excel changes the order they appear in that list.

Fixes #457

Further improve read performance

Instead of re-encoding the full sheet to change the namespaces
in the encoded bytes, read the sheet again and do the byte
replacements there.

In this case, file access ends up being more performant than
marshaling the sheet back to XML.

In the SharedStrings test, ensure the strings are actually read.

Fix #439

Update comments

Bugfix #454

Fix #439 

We were parsing the whole sheet twice since the
sheet reader already reads in all the rows.

getTotalRowsCols function is unused after these changes
so it has been deleted as well.

Closes #439

Fix #443

Fix #442

Add a missing element of the comment text elements

SetPageLayout support to set fit to width and height

New functions `GroupSheets` and `UngroupSheets` added
Refactor sheet index calculation