Identify candidates for deprecation in the fracs subfolder

[keileg]

Depending on confidence, outdated, unused, or otherwise hopeless code can be:

1. Marked for deprecation (see template below),
2. Brought up for discussion in the core developer group (or a subset). Please make a list of discussion points, if any, in this issue, with links to the relevant part of the code.
3. In extreme cases, simply deleted.

How to mark for deprecation:
``python
import warnings

...
msg = "This functionality is deprecated and will be removed in a future version"
warnings.warn(msg, DeprecationWarning)

For examples, search for 'deprecation' in the code base.

[keileg]

Import of fracture information from a .fab format will be deprecated.

Beyond this, it is clear that the whole folder is ripe for a full restructuring, but that will be a major undertaking that cannot be prioritized for now.