Replace the C-wrapped library ERFA.jl with Astrometry.jl's SOFA submodule #77

The package Astrometry.jl has a pure Julia submodule called SOFA that is a drop-in replacement for SOFA.jl and ERFA.jl. It is a high performance package with most functions having no allocations. The SOFA submodule was implemented to ensure the accuracy of the independent Astrometry.jl module. It would be best for both modules to be their own independent packages.

This issue is a request to replace the wrapped liberfa.c library with the pure Julia one and to tag it as version 2.0.0. What say you?