release/v2.15.0

Bugfix/issue 487 hypergeometric functions

Fix lint errors and allow zero numerator coefficients in hypergeometric functions to lead to polynomial.

Feature/issue 512 speedup ode

Feature/issue 1452 new to matrix signatures

Adjusted tolerance 1.05e-6 vs. 1e-6 on 3rd order diff test that auto-diffs grad_2F1.  grad_2F1 might need a non-autodiff gradient for this to be more accurate in the future.

Up requested accuracy in tests so current code fails, replace current code with trimmed down copy of 3F2 gradient code. Tests pass at 1e-8. [no ci]

Merge branch 'bugfix/issue-487-hypergeometric-functions' of github.com:stan-dev/math into bugfix/issue-487-hypergeometric-functions

Huh?

Reverted precision request to default and used while(true) loop as there are multiple exit conditions.

Merge pull request #510 from stan-dev/feature/issue-38-univariate-normal-distribution-on-sufficient-statistics

Feature/issue 38 univariate normal distribution on sufficient statistics