Use sigma instead of sd

[MarcoGorelli]

e.g. Normal('x', mu=mu, sigma=sigma) rather than Normal('x', mu=mu, sd=sigma)

sd will (silently) continue working, see pymc-devs/pymc#4344

[CloudChaoszero]

Hey @MarcoGorelli, I am working on some big changes related to switching from pymc3 plots to arviz plots.

• Being said, I can take this on as well, and work on both of them.

[twiecki]

That's great @CloudChaoszero - much appreciated!

[CloudChaoszero]

I updated the PR to include replacing sd with sigma parameter.

• If any lingering ones exist, we can update it over time, I imagine.