Allow for the user to override the templates in GenericFileIOCalculators

The new GenericFileIOCalculators have a lot of modularity, including the ability (in principle) to customize the I/O processes of the calculator. The I/O is often taken care of via a template object, but this template is hard-coded in all GenericFileIOCalculators. Ideally, just like with the profile, we should allow for (expert) users to override the template with their own custom template if they feel so inclined.

@askhl: What are your thoughts on this? If you think it is reasonable, I will clean up this MR (e.g. add docstrings). If there was another way one was supposed to do this, please let me know.

Checklist

• I am familiar with ASE's contribution guidelines.
• [] Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.