Updated comments to persist dimensions on write.

Created originally on Bitbucket by mgemmill (Mark Gemmill)

Was already merged in Bitbucket before import, marked as merged by the import user

I put this PR together in response to issue #978 (closed), which was something I was looking for as well.

https://bitbucket.org/openpyxl/openpyxl/issues/978/change-width-and-height-in-classcomment