Adds `set_strain_data_from...` functions to allow easy setting of the
data.

Dynesty checkpointing

See merge request Monash/tupak!74

Add test for prior

Closes #88

See merge request Monash/tupak!76

Most of these tests fail because they rely on the old set_data method,
or try to set attributes that aren't settable.

The tupak.core.prior call in the core/__init__.py file was accidently
removed during testing. This puts it back in.

If the prior is not given to the likelihood when using the marginalized
likelihood, marginalization cannot be safely performed. This adds a
check and raises a ValueError if it is not set.

Closes #88

- Adds in comments from Denyz, and general tidy up.
- Closes #83

Also fixes a typo