Stochastic dynamics framework

[FriesischScott]

This PR adds basic support for stochastic processes through the spectral representation method. More stypes of stochastic processes such as K-L expansion will follow later.

The process is represented and sampled through the internal random variables. This allows us to use advanced MC methods among other things. For convenience, we include a StochasticProcessModel (working title) that a user can use to compute the timeseries and add it to the DataFrame for later models.

There will be more to come and sufficient documentation before the merge.

[FriesischScott]

The extra deps are just for testing.

[FriesischScott]

We're trying to put together a proper example using a model of a 42 story building in OpenSees.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 1.42857% with 69 lines in your changes missing coverage. Please review.

Project coverage is 90.45%. Comparing base (c350a49) to head (2ae5a4b).

Files with missing lines	Patch %	Lines
...puts/stochasticprocesses/spectralrepresentation.jl	0.00%	29 Missing :warning:
src/util/fourier-transform.jl	0.00%	22 Missing :warning:
src/dynamics/psd.jl	0.00%	8 Missing :warning:
src/inputs/stochasticprocesses/models.jl	0.00%	6 Missing :warning:
src/models/external/externalmodel.jl	0.00%	4 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #180      +/-   ##
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- Coverage   94.49%   90.45%   -4.04%     
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  Files          35       39       +4     
  Lines        1545     1614      +69     
==========================================
  Hits         1460     1460              
- Misses         85      154      +69     

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