* Rewrite prepareSheetXML to scale linearly
We don't need to backfill columns into every row for most purposes
Provided makeContiguousColumns for setting styles where we do
need it for a specific region.
Added a benchmark to monitor progress. For 50,000 rows this went
from about 11 seconds to 1 second. The improvements are more
dramatic as the row/column count increases.
* Assigning that row value was redundant
* 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
Use github.com/mohae/deepcopy to deep copy worksheets instead of the
internal deepcopy function that was using encoding/gob serialization and
deserialization.
Rationale:
1/ using `encoding/gob` is much slower than
[`mohae/deepcopy`](https://github.com/mohae/deepcopy/)
2/ When building an application this implementation of `deepcopy` drags
the `encoding/gob` package into the binary. And this package is much
bigger than `mohae/deepcopy` (which only depends on `time` and `reflect`).
```
$ LC_ALL=C stat -f "%6z %N" $(go env GOPATH)/pkg/$(go env GOOS)_$(go env GOARCH)/github.com/mohae/deepcopy.a $(go env GOROOT)/pkg/$(go env GOOS)_$(go env GOARCH)/encoding/gob.a
10508 .../pkg/darwin_amd64/github.com/mohae/deepcopy.a
541818 .../pkg/darwin_amd64/encoding/gob.a
```
* Add information about return value of NewSheet()
* Minor documentation language fixes
Samples:
* Added sample go file for dumping a XLSX file to the console
xuri