Merge pull request sirocco-rt#633 from agnwinds/dev

Pulling `dev` into `master` to create an official v84 release.

Merge branch 'dev' of https://github.com/agnwinds/python into dev

Merge pull request sirocco-rt#313 from agnwinds/import

Capability to Import Models

Merge pull request sirocco-rt#293 [LT2] from agnwinds/low_temp2

LT2: major changes, to pull in the work done by NSH and KSL in the summer of 2017 to address problems with python working in optically thick conditions.

This is Python 81c: Pre LT2 merge

This is Python 81a

This is Python 81.

changed version

Merge commit '4419dc20cdf08d64e1d6aa43b54aca9ef2b4504f' into release

added SPEC_MOD_FAIL to enum, see sirocco-rt#162