Add Langevin simulator for more general splittings

[noeblassel]

Add support for more general splitting schemes for Langevin dynamics.

Changes

• [x] Add LangevinSplitting type and simulate! method to simulate.jl
• [x] Add docstring describing the simulator
• [x] Write test comparing the current Langevin and LangevinSplitting with identical RNG, in the case of a BAOA splitting.

[codecov[bot]]

Codecov Report

Merging #67 (b7787d6) into master (2e35605) will increase coverage by 0.20%. The diff coverage is 98.14%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   86.73%   86.93%   +0.20%     
==========================================
  Files          28       28              
  Lines        2924     2978      +54     
==========================================
+ Hits         2536     2589      +53     
- Misses        388      389       +1     

Impacted Files	Coverage Δ
src/simulators.jl	99.41% <98.14%> (-0.59%)	:arrow_down:

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[jgreener64]

:+1: