Start working on CURAND 4.1 features.

It has 2 new generators. One is very similar to XORWOW so it should be
possible to introduce inheritance like for Sobol. Second generator
is different (requiring different initialisation and introducing
limitations, e.g. number of threads that can use it) but presents
the same device API.
Extract _PseudoRandomGeneratorBase class.

Extracted class contains code shared by XORWOW and MRG23k3a generators.
Those two generators share most of the code; MRG23k3a allows for
skipping subsequences.
Work on MRG32k3a

Fix name of function preventing compilation of wrapper.
Start adding documentation.
Work on Poisson distribution

Fix some spelling mistakes.
Start adding device-side Poisson distribution generation.
Finish Poisson random number generators

Add documentation, ensure that Poisson-related functions
are compiled only on CURAND 5.0 and above, add tests
Ensure that correct integer type is passed

Some CURAND functions expect 64-bit integers (unsigned long long),
not unsigned int. This is the case for generating uniform values,
and skipahead calls.
Skeleton for Mtgp32-specific functions.
Small fixes in CURAND documentation.
Remove not-yet-fully-implemented Mtgp generator.
Remove documentation for missing Mtgp generator.
Do not import removed functions.
Nuke a few tabs that snuck in
Merge branch 'curand-41'