Rethink/cleanup FATES hybrid logic

[samsrabin]

I was expecting the top part of this logical block to also exclude hybrid runs along with branch runs. Is a hybrid run equivalent to is_restart()=.true. ?

For future work I feel like we should clean some of this logic up, I cobbled a lot of this together many years ago when I did not have a good sense of restart options. But my take is that our decision making on how to restart fates (ie cold or from pre-existing vegetation structure in a restart file) really comes down to if it is a startup, branch or hybrid. The finidat string is less straight forward to use here in decide what part of the initialization sequence to use. My understanding is that the finidat should have a meaningful file path only in the latter two scenarios (branch and hybrid), so its better to use the switches that are the root of deciding these things rather variables like finidat that are "downstream" of them. Does that make sense? Again, I'd like to make an issue of this and flag it for future work and not complicate this PR.

Originally posted by @rgknox in #2955 (comment)