Change `isxxx` and `hasxxx` to `is_xxx` and `has_xxx` (issue 1061).

The coding and design conventions (CDC) document was also updated.
Add entries to .gitignore file
Fix typos in docstrings
Change `isfront` and `isback` to `is_front` and `is_back`
Rename `center` to `centre` everywhere.

(I chose `centre` because there were more `centre` than `center`
around.)