Use `Number` instead of `Real` for `mgf` and `cf`

[theogf]

As pointed out in #1502 , mgf and cf should be allowed to take Complex numbers and the second argument should be therefore relaxed. All functions are also made type-stable. Breaking?, when the argument for the mgf is given outside of the domain definition a DomainError is thrown.

[theogf]

Thanks for the monster review! I tried to address your comments and integrate them. The inference tests should tell us where things go wrong :)

[theogf]

MGF are not necessarily defined on the whole Real/Complex domain, what should be returned when getting a value out of bound? NaN or ArgumentError ?

[devmotion]

MGF are not necessarily defined on the whole Real/Complex domain, what should be returned when getting a value out of bound? NaN or ArgumentError ?

I was wondering the same. I'm a bit unsure, do you have a concrete example? Then it might be easier to see.

[codecov-commenter]

Codecov Report

Merging #1504 (186066a) into master (620a73a) will increase coverage by 0.04%. The diff coverage is 80.00%.

:exclamation: Current head 186066a differs from pull request most recent head 9a1211d. Consider uploading reports for the commit 9a1211d to get more accurate results

@@            Coverage Diff             @@
##           master    #1504      +/-   ##
==========================================
+ Coverage   84.92%   84.97%   +0.04%     
==========================================
  Files         128      128              
  Lines        7735     7765      +30     
==========================================
+ Hits         6569     6598      +29     
- Misses       1166     1167       +1     

Impacted Files	Coverage Δ
src/Distributions.jl	100.00% <ø> (ø)
src/univariate/continuous/biweight.jl	61.76% <0.00%> (-1.88%)	:arrow_down:
src/univariate/continuous/epanechnikov.jl	60.52% <0.00%> (-1.64%)	:arrow_down:
src/univariate/continuous/triweight.jl	52.38% <0.00%> (-7.08%)	:arrow_down:
src/univariate/continuous/cauchy.jl	85.10% <100.00%> (+0.66%)	:arrow_up:
src/univariate/continuous/chisq.jl	76.66% <100.00%> (+1.66%)	:arrow_up:
src/univariate/continuous/erlang.jl	69.44% <100.00%> (+1.79%)	:arrow_up:
src/univariate/continuous/exponential.jl	94.00% <100.00%> (+0.25%)	:arrow_up:
src/univariate/continuous/gamma.jl	94.38% <100.00%> (+0.12%)	:arrow_up:
src/univariate/continuous/inversegamma.jl	96.00% <100.00%> (ø)
... and 26 more

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[theogf]

This has gone very very stale so I will just close it.