Added min model error options, incl "by site"

[brendan-m-murphy]

Also added "robust" option to use median absolute deviation in place of standard deviation.

• Summary of changes

In an ini file use:

compute_min_error = True
min_error_options = {"by_site": True, "robust": True, "average_over": "by_site"} 

to use new options. (Either "by_site" or "robust" can be omitted; or min_error_options can be omitted entirely)

• Please check if the PR fulfills these requirements

• [ ] Closes #xxxx (Replace xxxx with the Github issue number)

• [ ] Tests added and passing

• [ ] Documentation and tutorials updated/added

• [ ] Added an entry to the CHANGELOG.md file

• [ ] Added any new requirements to requirements.txt

[joe-pitt]

Adding an option for a percentile-based calculation on min_model_error. This changes what is required in the ini - calculate_min_error now takes either "percentile" or "residual". Need to update docs/templates to reflect this

[joe-pitt]

One thing that has just occurred to me - do we need to raise an error in the case when calculate_min_error is neither "percentile", "residual" or None? I guess at the moment if someone gets the spelling wrong it would silently skip the min error calculation