Accept keywords after comma in parametric model macros

[pablosanjose]

Closes #118

We also reduce code repetition in macros by condensing all functionality into a single parse_term function

(Note that @onsite(r -> 2, sublats) is also accepted if sublats has been defined in scope, as in common functions)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.53%. Comparing base (55f1659) to head (481963f).
Report is 10 commits behind head on master.

Files	Patch %	Lines
src/models.jl	92.30%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
- Coverage   92.54%   92.53%   -0.01%     
==========================================
  Files          37       37              
  Lines        6100     6106       +6     
==========================================
+ Hits         5645     5650       +5     
- Misses        455      456       +1     

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