Added mle for weibull distribution

[rory-humphries]

I added an implementation that solves for the Weibull maximum likelihood estimators using newtons method. Included some basic tests as sanity checks and updated the docs.

I noticed that there was some discussion in #1267 on whether or not use a generic newtons method and wondering if that's been decided on.

Merging this can close #702.

[devmotion]

Thank you! I added some comments and suggestions.

[codecov-commenter]

Codecov Report

Merging #1342 (e22ba10) into master (5cafbcd) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1342      +/-   ##
==========================================
+ Coverage   82.16%   82.23%   +0.06%     
==========================================
  Files         116      116              
  Lines        6604     6635      +31     
==========================================
+ Hits         5426     5456      +30     
- Misses       1178     1179       +1     

Impacted Files	Coverage Δ
src/univariate/continuous/weibull.jl	93.40% <100.00%> (+2.78%)	⬆️
src/common.jl	65.71% <0.00%> (-1.86%)	⬇️
src/univariates.jl	61.71% <0.00%> (-0.49%)	⬇️
src/univariate/locationscale.jl	92.30% <0.00%> (+0.47%)	⬆️
src/univariate/discrete/dirac.jl	94.44% <0.00%> (+0.69%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5cafbcd...e22ba10. Read the comment docs.

[devmotion]

Looks good now, also the formulas seem to be correct 👍 I added two additional minor suggestions.

[devmotion]

Thanks a lot!