BUG: Fix bug in Skew T where RandomState was ignored

[bashtage]

Switch to user-provided RandomState
Add exxample of simulation
Add tests of rescaling

[codecov]

Codecov Report

Merging #353 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #353     +/-   ##
=========================================
+ Coverage   99.17%   99.27%   +0.1%     
=========================================
  Files          51       52      +1     
  Lines       11451    11523     +72     
  Branches      987      987             
=========================================
+ Hits        11356    11440     +84     
+ Misses         41       30     -11     
+ Partials       54       53      -1

Impacted Files	Coverage Δ
arch/univariate/distribution.py	100% <100%> (ø)	⬆️
arch/tests/univariate/test_rescale.py	100% <100%> (ø)
arch/tests/unitroot/test_unitroot.py	100% <0%> (ø)	⬆️
arch/tests/utility/test_cov.py	100% <0%> (ø)	⬆️
arch/tests/utility/test_timeseries.py	100% <0%> (ø)	⬆️
arch/tests/univariate/test_moment.py	100% <0%> (ø)	⬆️
arch/tests/bootstrap/test_multiple_comparison.py	100% <0%> (ø)	⬆️
arch/tests/univariate/test_distribution.py	100% <0%> (ø)	⬆️
arch/tests/univariate/test_volatility.py	100% <0%> (ø)	⬆️
arch/tests/test_data.py	100% <0%> (ø)	⬆️
... and 9 more

---

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 473431c...622d064. Read the comment docs.

[coveralls]

Coverage decreased (-0.1%) to 99.491% when pulling 622d064 on sim-doc into 473431c on master.