[BUG] fix incorrect scale parameter in Distribution

[dswah]

Fixes #370

• Add tests like proposed in Incorrect Scale Parameter in Distribution #370
• tests check scale, and of course log-likelihood

[dswah]

@serjisa @jamesdeeel
Could one of you give a review of the fixes here?

[serjisa]

Hi!

Yeah, quick check shows that now pyGAM gives reasonable log-likelihood estimations that are consistent with statsmodels: https://colab.research.google.com/drive/1gYm4FES72bClRzRu4G9Jnv0xeLdD0sNW?usp=sharing

Thank you a lot!