Gemini3d

Better auto-detection of compatible compilers. This is especially useful for MacOS where clang masquerades as GCC.

bugfixes

* check_finite_output checks on all images, not just root (more robust early failure when a sim doesn't converge)
* correct linking of Mumps-Scalapack-Lapack--remove linking hacks, use modern CMake INTERFACE to eliminate redundant and buggy duplicate link specifications
* improve PyGemini interface robustness to use `python -m gemini.<method>`
* fix CMake infinite loop when called from MatGemini

Features

* select Fang 2008 (maxwellian) or Fang 2010 (monoenergetic) models
* improve test parallelism

While normally new users or new systems should have all self-tests run, to save build time we add the option to disable many self-tests with `cmake -DBUILD_TESTING=off`. If any problem is experienced, it's best to use self-tests as they tell more specifically where the problem is occurring.

HDF5 finding behavior improved.

further robustness improvements, make API more like MatGemini for user convenience.

* add self tests
* give better user feedback when a library is missing or not working