Bug norm expan shapes

NormExpan has incorrect shapes with breaks with recent scipy.

This is a slightly different solution, than #1397 by @piannucci
(I guess it avoids args introspection in scipy)

plus some basic unittests for NormExpan. 
I didn't find a Gram-Charlier expansion distribution in R seek. (either it doesn't exist or it has a different name)

plus small cleanup: move examples out of module

test coverage for the expansion is still low
I tried a few more things to check whether the results are roughly correct, limit on RMSE, check for fat tails if kurtosis > 0, check for asymmetry if skew > 0, but haven't added them. They would be very coarse checks that qualitatively the results look right.

related: Evgeni has Edgeworth Expansion in PR #1325 with more complete, explicit implementation of other distribution methods.