Deprecate SuperLU

[spdomin]

SuperLU is only used for a coarse direct solve for Muelu's AMG approach. As such, the standard KLU build in Trilinos works perfectly fine.

Lets:

1. Deprecate SuperLU
2. Modify the manual build instructions to say that SuperLU is optional with KLU available if the Muelu XML file includes "KLU" (for a Trilinos install that includes SuperLU).
3. Accept the diffs for gcc 8.3/mpi4.0.3

[spdomin]

This is the Mulue XML note that we should add if a user has a SuperLU Trilinos install, however, wants to use KLU:

<Parameter name="coarse: type" type="string" value="klu2"/>

[spdomin]

Basically, the premise here is that if a Nalu build used a version of Trilinos that did not include SuperLU, then KLU is the automatic default.

[spdomin]

Done - document is up-to-date as well.

170349b