Update Doxygen documentation

[smpark7]

This issue can be closed when the Doxygen documentation in Moltres is updated to reflect the latest Moltres build.

The goal is to update the Doxygen generated html files for arfc.github.io/moltres, which were last compiled in 2017, so that the latest kernels and other objects are accounted for.

This is separate from (but somewhat related to) issue #133, which highlights missing docstrings for functions and variables in .h files. Ideally, we would close #133 before closing this issue.

[smpark7]

@yardasol How's the progress with #133? I was thinking of just updating the Doxygen files first and then running another update after #133 is resolved

[yardasol]

@smpark7 I started a list of files back in October in order to determine which ones needed attention. Most of it is not currently filled out.

[yardasol]

@smpark7 Are you updating the existing doxygen files or are you adding new ones as well?

[smpark7]

Both. Doxygen updates existing files if there are any changes, and creates new ones for kernels/objects that didn't exist before. I'll go ahead and make a PR because a Doxygen update is long overdue. We can update it again whenever needed