added MPI parallelization among wavelength grid
MPI: added broadcast of all wavelength-integrated values so everybody is on the same page
YAML: increased default number of wavelength points to 1000 in sample input file
YAML: wrote default wavelength limits in Angstrom-friendly format in sample input file
HDF: added parallel HDF dump of emergent spectrum

Each process holds a different part of the wavelength grid. Ultimately
it will be written to a file so rather than create a MPI data structure,
gathering the data from all the workers and then writing it to file with
just one node, we can instead have each worker write its own section to
an HDF file using parallel HDF I/O.
Merge branch 'add_MPI' into develop

Conflicts:
	src/CMakeLists.txt