GPU support by adding CuParticles

[baggepinnen]

This PR adds a type CuParticles that works as regular Particles but is backed by a CuArray. Expect in the range of 5-7x speedup over Particles if the sample size is 1e5 - 1e7.

EDIT 2021-04-22: For complex functions, the benefit of CuParticles may appear already above 1e3 particles.

Future memory-management improvements to the GPU ecosystem is likely to improve upon this number.

[codecov]

Codecov Report

❌ Patch coverage is 88.32117% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.73%. Comparing base (fb61791) to head (f42a6e7).
⚠️ Report is 168 commits behind head on master.

Files with missing lines	Patch %	Lines
src/plotting.jl	87.01%	10 Missing ⚠️
src/particles.jl	85.00%	3 Missing ⚠️
src/types.jl	0.00%	2 Missing ⚠️
src/MonteCarloMeasurements.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   94.27%   93.73%   -0.54%     
==========================================
  Files          18       19       +1     
  Lines         891      990      +99     
==========================================
+ Hits          840      928      +88     
- Misses         51       62      +11     

Flag	Coverage Δ
unittests	93.73% <88.32%> (?)

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