Collection of depletion-related fixes and enhancements

[paulromano]

I recently noticed that a few of the equations listed in the docstrings for the CF4Integrator, CELIIntegrator, and EPCRK4Integrator classes were not exactly right. This PR fixes the equations.

Edit: I've added a bunch more fixes/enhancements related to depletion:

• Integrator.integrate now has some minimal output that gives the user some feedback about the depletion timesteps and powers being used
• The AtomNumber class now has a get_atom_densities method that gives you a dictionary mapping nuclides to densities for a given material -- this is the easiest way to see what the state of the class is for a given material. Also, I've made it so that you can pass a Material object directly rather than (awkwardly) just a string of the ID.
• Added a __repr__ method for the deplete.Nuclide class that gives some basic info
• Fixed a bug with diff_burnable_mats whereby materials were not getting correctly assigned when this flag was on
• I was noticing some weird output buffering issues when running depletion which are now fixed by the change in output.cpp

[shimwell]

Thanks for these timely improvements, I'm taking a look now and will run things tomorrow morning

[shimwell]

This looks good to merge to me, there is one optional change which is mainly due to my own bias towards f strings in all cases 😄 but either option is fine.

The addition of get_atom_densities adds a nice amount of convenience for the user 👍

The tiding up of the material id number using the private method _get_mat_index is also nice.

Tests pass and if we merge soon then this could be ready for the upcoming workshop which would be useful