Convert ExaCA to use Mist files for material properties

[gknapp1]

Currently, ExaCA has a set of material files that it pulls from in order to set material properties (resources/ExaCA/materials). To be consistent with other applications, it should ultimately use Mist files for updating material settings.

However, this depends on the ability of Mist to output ExaCA-compatible input files, so that capability needs to be implemented first. The related issue on Mist is ORNL-MDF/mist#27.

[gknapp1]

Opened a PR on the Mist repo to add ExaCA material file generation: ORNL-MDF/mist#30