Adds extra var matrix block methods

[SteveBronder]

Summary

This adds a few things I need in var<matrix> and some type traits for checking the inner types of var<Eigen> types.

Essentially with these specialized block member functions I can reuse a whole bunch of the code I just wrote for rvalue() and assign() which is nice!

Tests

Tests added for checking new block member functions propogate adjoints correctly and slice values correctly. Added simple tests for the type traits

Side Effects

Nope!

Release notes

Add more methods for taking blocks of var matrices.

Checklist

• Math issue How to add static matrix? #1805

• Copyright holder: (fill in copyright holder information)

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the 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/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[bbbales2]

Review

[bbbales2]

Add x.adj().setZero(); if you decide it's important to keep the is_nan branch here. The previous entries of x should be unaffected by the NaN-ness of whatever replaces it.

[SteveBronder]

I removed the nan check and just wrote a separate function the compiler can use

[bbbales2]

One change. Looks good.

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.02	3.64	0.83	-20.69% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	1.02	2.16% faster
eight_schools/eight_schools.stan	0.12	0.12	0.96	-4.04% slower
gp_regr/gp_regr.stan	0.17	0.17	1.0	-0.05% slower
irt_2pl/irt_2pl.stan	5.7	5.78	0.99	-1.3% slower
performance.compilation	86.51	85.59	1.01	1.06% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.47	8.65	0.98	-2.15% slower
pkpd/one_comp_mm_elim_abs.stan	29.8	29.44	1.01	1.22% faster
sir/sir.stan	136.84	133.4	1.03	2.52% faster
gp_regr/gen_gp_data.stan	0.04	0.04	0.98	-2.01% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.96	3.0	0.98	-1.62% slower
pkpd/sim_one_comp_mm_elim_abs.stan	0.38	0.38	1.01	0.5% faster
arK/arK.stan	1.77	1.8	0.99	-1.15% slower
arma/arma.stan	0.6	0.6	1.0	0.01% faster
garch/garch.stan	0.74	0.59	1.26	20.5% faster
Mean result: 1.00280885786

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Jenkins Console Log
Blue Ocean
Commit hash: 3971748

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Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix