Make overwriting/not overwriting existing files explicit

[ntessore]

Currently, functions like write_cls(), write_mms(), etc. have a default clobber=False parameter, meaning that new data is appended to existing files. This can be surprising.

Instead, we should have a default setting of clobber=None, which raises an error if an existing file is encountered. The user then has to specify clobber=True or clobber=False explicitly.