Add .test for Normal distribution

[jykr]

Observation modeled to have error with Normal distribution with mean field error and identity link function. i.e.,

$$ y=X\beta + a + \epsilon $$

$$ \epsilon \sim \mathcal{N}(0, \mathrm{diag}(\vec{\sigma^2})) $$

[ilia-kats]

Awesome, thanks for this. Looks good in general (I've only had a cursory look at the _do_test implementation, I trust that it's correct), but a few things aren't clear to me (see the specific comments).

[ilia-kats]

Looks good now. Could you just run the black formatter on the code?

[ilia-kats]

Great, thank you.

[jykr]

Awesome! Thank you!