Add sum_to_zero_matrix type

[WardBrian]

Closes #1505. Needs https://github.com/stan-dev/math/pull/3169 and https://github.com/stan-dev/stan/pull/3342

Submission Checklist

• [x] Run unit tests
• Documentation
  • [x] If a user-facing facing change was made, the documentation PR is here: Issue https://github.com/stan-dev/docs/issues/867

Release notes

Added a new constraint, sum_to_zero_matrix, which is a matrix that sums to zero along both axes.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.

Project coverage is 89.56%. Comparing base (e511e97) to head (66ab723). Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
src/frontend/Ast_to_Mir.ml	80.76%	5 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1506   +/-   ##
=======================================
  Coverage   89.55%   89.56%           
=======================================
  Files          66       66           
  Lines        9695     9694    -1     
=======================================
  Hits         8682     8682           
+ Misses       1013     1012    -1     

Files with missing lines	Coverage Δ
src/frontend/Pretty_printing.ml	92.30% <100.00%> (+0.02%)	:arrow_up:
src/frontend/Ast_to_Mir.ml	93.90% <80.76%> (+0.17%)	:arrow_up:

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