Dynamic template fetching deprecation

[TheChymera]

https://github.com/IBT-FMI/SAMRI/blob/master/samri/fetch/templates.py#L22

We should look to migrate our template resolution to proper package management (instead of some web download at runtime), similarly to how this was done for the mouse brain templates:

• Template generator package.
• Template package atom for e.g. Gentoo.
• Dependency specification according to PMS.