Reorganize reference frame module and add more transformations (#105)

This moves the hard coded transformation parameters to their own source
file in an attempt keep things clean. It also adds many more
transformations (primarily ITRF2020 to older ITRF versions, and from
ITRF to the matching ETRF version). Finally a couple of structs and
functions are marked as `pub` to allow others to implement their own
transformation parameters instead of relying on own hard coded set of
transformations only.

Author: jbangelo