Add Exponential Distribution quantile function

[andrjohns]

Summary

This PR introduces the quantile function of the exponential distribution.

I've used the _qf suffix, as suggested by @spinkney over in #2513

Tests

prim and mix tests have been added

Side Effects

Additional prob folders have been created in the fwd and rev folders so that the gradients for vector returns can be specified.

Release notes

Added quantile function for the exponential distribution

Checklist

• Math issue Add distribution quantile functions (inverse CDFs) #2517

• Copyright holder: Andrew Johnson

  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

[andrjohns]

Note that the rest of the quantile functions to be added will follow the same naming _qf suffix and folder structure rev/prob and fwd/prob, so I'd encourage anyone interested to speak up if that clashes in any way

(Let me know if there's anyone I should tag to make sure this gets seen)

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.51	3.54	0.99	-0.83% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.99	-0.88% slower
eight_schools/eight_schools.stan	0.09	0.08	1.02	1.89% faster
gp_regr/gp_regr.stan	0.15	0.15	0.98	-2.35% slower
irt_2pl/irt_2pl.stan	5.78	5.73	1.01	0.84% faster
performance.compilation	93.67	91.23	1.03	2.61% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.34	8.13	1.03	2.49% faster
pkpd/one_comp_mm_elim_abs.stan	32.95	31.6	1.04	4.1% faster
sir/sir.stan	117.98	117.05	1.01	0.79% faster
gp_regr/gen_gp_data.stan	0.04	0.04	0.97	-3.31% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.96	3.01	0.99	-1.45% slower
pkpd/sim_one_comp_mm_elim_abs.stan	0.4	0.39	1.02	2.16% faster
arK/arK.stan	2.08	2.05	1.01	1.43% faster
arma/arma.stan	0.27	0.27	1.0	0.25% faster
garch/garch.stan	0.61	0.61	1.0	-0.17% slower
Mean result: 1.0054449601

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Jenkins Console Log
Blue Ocean
Commit hash: cf33e69

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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

[bob-carpenter]

I've used the _qf suffix

The suffix should match whatever we did for the normal inverse cdf. But all I can find is inv_Phi. If this is the first general one, I'm OK with _qf.

[andrjohns]

Closing for now to clear PR backlog, will re-open once ready for review