Rework ProbabilityBox and Interval

[FriesischScott]

This PR introduces two main changes:

1. The ProbabilityBox is now without name and has to be wrapped inside a RandomVariable (now a parametric type)

• Additionally, a pbox is now constructed using a Dictionary mapping the parameters of the distribution to either a Real or a Interval
• I think this is a lot cleaner than before and also easier to handle in the code.

3. The Interval type is now mainly used internally for the propagation of epistemic uncertainties and to construct p-boxes

• A new IntervalVariable is introduced to be used as inputs

This gives us more flexibility and allows us to use intervals and pboxes inside other structs in downstream packages.

As an added benefit this gives us dependent pboxes.

I've also added documentation on imprecise reliability analysis.

Closes #196

[codecov]

Codecov Report

Attention: Patch coverage is 90.99099% with 10 lines in your changes missing coverage. Please review.

Project coverage is 94.21%. Comparing base (f5ee6cc) to head (ec89af8).

Files with missing lines	Patch %	Lines
src/inputs/imprecise/p-box.jl	84.84%	5 Missing ⚠️
src/inputs/imprecise/interval.jl	88.46%	3 Missing ⚠️
src/inputs/jointdistribution.jl	85.71%	1 Missing ⚠️
src/reliability/probabilityoffailure_imprecise.jl	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
- Coverage   94.30%   94.21%   -0.09%     
==========================================
  Files          42       43       +1     
  Lines        1772     1799      +27     
==========================================
+ Hits         1671     1695      +24     
- Misses        101      104       +3     

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