Rename Tguess in PTE solver and document temp[0] usage

[jhp-lanl]

In our PTE solvers, we actually use temp[0] as the starting point for the sovler (if I'm reading our code correctly). Tguess seems to really just be a ballpark value used to rescale the temperatures for numerical niceness.

I think we need to

1. document the usage of temp[0] in our PTE solvers and note that the PTE solver benefits from a sane value
2. rename Tguess to something that more appropriately describes its function

[jhp-lanl]

I didn't realize you were already working on this @Yurlungur ! Thanks for your MR addressing this!

[Yurlungur]

I saw your issue yesterday and I was already playing in the PTE solver for other reasons, so I just added it. :)