Adds size checking to uni index array assignment

[SteveBronder]

Submission Checklist

• [x] Run unit tests: ./runTests.py src/test/unit
• [x] Run cpplint: make cpplint
• [x] Declare copyright holder and open-source license: see below

Summary

Fixes #3150 to do a size check when assigning to the inner elements of an array whose inner elements also have dimensions.

Intended Effect

Stop the use of accidental ragged arrays

How to Verify

New test added that can be run with

python ./runTests.py -j18 ./src/test/unit/model/indexing/assign_test.cpp

Side Effects

Documentation

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)