Minor improvement: Change to_excel "sheet_name" to "sheetname"

[saintsfan342000]

pd.read_excel() takes "sheetname" as an argument.
pd.DataFrame.to_excel() takes "sheet_name" as an argument.

For consistency, I think one ought to be changed to match the other.
On my windows installation of pandas 0.16.2, I modified to_excel to take sheetname rather than sheet_name. All I had to do was locate frame.py within the core directory. to_excel() is defined here, and sheet_name is written twice in the function and once in the docstring.

Very new to github so if ideas like these are more appropriately filed through PRs I apologize.

[jorisvandenbossche]

Agree we should make them the same (at least make it available, maybe not necessary to actually deprecate the discouraged alias).

I am only not fully sure if would go with sheetname instead of sheet_name (we are not always very consistent in the use of _ to separate words in keywords, but I think the general policy is to use them).

And @saintsfan342000, opening an issue for this is very good! If you get approval, always welcome to open a PR afterwards!

[abarber4gh]

current (0.20.x) branch appears to mostly use sheet_name so propose changing sheetname to sheet_name.

$ git status
On branch 0.20.x

49 occurrences of sheet_name in io

$ grep -rc sheet_name pandas/io | grep -v 0
pandas/io/excel.py:42
pandas/io/formats/excel.py:3
pandas/io/formats/style.py:2

25 occurrences of sheetname in io

$ grep -rc sheetname pandas/io | grep -v 0
pandas/io/excel.py:25