port MHD from Nyx

[zingale]

the MHD port should be straightforward. Some thoughts:

• we don't need any cosmological expansion factors
• we need to ensure that it works with a general EOS
• can we hook in radiation too? it may be straightforward with MOL and reusing the implicit solve
• can we still do the same conservative gravity?

[zingale]

Here's some status:

• There are a number of places where gamma_const and gamma_minus_one appear -- I've marked these with comments beginning with ! TODO along with my thoughts on how to fix (usually calling an EOS)

• For the transverse update and Riemann, this does not do the method we use in hydro of separately evolving (rho e) and p. For now, we'll just use the EOS to make everything we need

• species seem to be incomplete. In particular, there are a number of places in the convert from primitive and conserved (and back) that don't seem to manage the density factors.

[zingale]

In Castro_mhd.cpp, the following todos remain:

• the source terms are not implemented
• the flux registers are not filled

[zingale]

There are a lot of places where we go between E and e in the code. We now need to make sure that all of these places include the magnetic energy as well, as:

rho E = rho e + K + 0.5 * B**2

Here's a list of places to change:

• enforce_consistent_e

[zingale]

MHD was merged in PR #307