Add `update()` call when importing templateflow

[mgxd]

To greatly simplify users updating their templateflow skeletons as templates are added / change.

• stick a call to templateflow.conf.update() in templateflow.__init__.py
• allow users to freeze current skeleton by setting environmental variable, i.e. TEMPLATEFLOW_AUTOUPDATE=0
• fail gracefully and inform users when trying and failing to update skeleton

[oesteban]

I would add to this:

• remove the post-installation hook

[oesteban]

BTW the call should be templateflow.conf.update(local=True, overwrite=False)

If a skeleton needs to be upgraded, then that should be done intentionally by the user (or fMRIPrep) with templateflow.conf.update(local=False, overwrite=False)

[mgxd]

If a skeleton needs to be upgraded, then that should be done intentionally by the user (or fMRIPrep) with templateflow.conf.update(local=False, overwrite=False)

This just bit me in nipreps/fmriprep-rodents@a6bd87f. It should be easy for users to ensure they are working with the latest templates, WDYT about adding a flag (maybe --fetch-latest-templates) to allow users to run an update with overwrite=True?

[mgxd]

Ah nevermind, this is #58