fixes #498, mdivide_left_tri_low now calls var overload

[bob-carpenter]

Submission Checklist

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

Summary:

Fix overload logic in mdivide_left_tri_low so that it calls the var version of mdivide_left_tri where appropriate.

Intended Effect:

Reduce memory size for matrix division.

How to Verify:

Unit tests still pass.

Side Effects:

May be a bit slower according to @rtrangucci

Documentation:

I added doc and tests for mdivide_left_tri_low showing how it's used as a view and gets the right values. Other tests for mdivide_left_tri remain.

Reviewer Suggestions:

Anyone.

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):

Columbia University

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/)

[rtrangucci]

I'm taking a look at this now

[rtrangucci]

@bob-carpenter This looks good. The new test only tests the var/var instantiation of the function, but I added the double/var and var/double versions. I'll push, and when those tests pass, we can merge this. Thanks Bob!

[bob-carpenter]

@rtrangucci Thanks for the extra tests. I didn't add derivative tests, either. Should we go through and test the derivatives are the same as for inverse(X) * Y?

Also, do we need to do anything for dividing on the right?