vectorize scenario generation, add LOTS of tests

[jason-ash]

Working on #50 to try to speed up scenario generation.

[jason-ash]

Rebased onto master at b9a042b

[jason-ash]

Pretty impressive performance 🏎️ !

It takes 0.16 seconds to generate 10,000 scenarios with 252 timesteps each using the BlackScholesProcess.

Python 3.6.7 |Anaconda custom (64-bit)| (default, Dec 10 2018, 20:35:02) [MSC v.1915 64 bit (AMD64)]
Type 'copyright', 'credits' or 'license' for more information
IPython 7.2.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from pyesg import BlackScholesProcess

In [2]: model = BlackScholesProcess.example()

In [3]: %timeit model.scenarios(100.0, dt=1/252, n_scenarios=10000, n_steps=252, random_state=42)
164 ms ± 1.02 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

[jason-ash]

Rebase onto master at f67da1e