Replace all instances of StatsError with custom error types or Option<T>

[FreezyLemon]

Closes #221, I think.

Added some new error types similar to the ones introduced in #265.

Some functions only got the simpler Option<T> return type if they only have one error condition. The functions in MultivariateNormal also have a simple Option return type because they're non-public, and it's expected that the preconditions are fulfilled on every call anyways. No need for fancy error messages (and can be easily changed if need be)

[codecov]

Codecov Report

Attention: Patch coverage is 81.42857% with 26 lines in your changes missing coverage. Please review.

Project coverage is 93.94%. Comparing base (1f8e00a) to head (e87cde0).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/function/beta.rs	84.88%	13 Missing ⚠️
src/function/gamma.rs	70.58%	5 Missing ⚠️
src/statistics/iter_statistics.rs	0.00%	4 Missing ⚠️
src/function/exponential.rs	57.14%	3 Missing ⚠️
src/distribution/multivariate_normal.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #284      +/-   ##
==========================================
+ Coverage   93.75%   93.94%   +0.19%     
==========================================
  Files          53       52       -1     
  Lines       11800    11783      -17     
==========================================
+ Hits        11063    11070       +7     
+ Misses        737      713      -24     

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[YeungOnion]

I would drop 9fa380a since we'll go with a panic only in #278 since it's internal.

[YeungOnion]

Wondering if it's valuable to emit a Result<f64, ValueWithConvergence{value: f64, rel_tol: f64}> to describe what convergence was like at the final iteration, or perhaps that would make sense as a part of returning values that emit precision as well as converged value.

[FreezyLemon]

I would drop 9fa380a since we'll go with a panic only in #278 since it's internal.

I'll leave this PR as-is for now and update it then.. If i revert any of the changes here then I also have to revert the removal of StatsError, which is kind of the end goal of this

Wondering if it's valuable to emit a Result<f64, ValueWithConvergence{value: f64, rel_tol: f64}> to describe what convergence was like at the final iteration

Maybe. I'd say it's a bit out of scope for this PR though, might make sense to open an issue for it

[YeungOnion]

I'll leave this PR as-is for now and update it then.. If i revert any of the changes here then I also have to revert the removal of StatsError, which is kind of the end goal of this

that makes sense, odd that deleting error.rs was a conflict, but I did the rebase locally and merged that last commit removing it.

[FreezyLemon]

@YeungOnion Not sure what happened, but error.rs was not deleted in the last commit. Maybe something went wrong during the rebase?

[YeungOnion]

Presumably, the commit I force pushed added it back into the tree 🙃