Create uses_of_utils.prior for better documentation

[SandhyaGiribabu]

Issue:
The utils.priors (code) have been introduced to streamline the placement of priors over model parameters as well as to simplify the incorporation of prior mean functions. Several examples (in docstrings and/or in a separate notebook) of their usage would be beneficial.

[matthewcarbone]

@SandhyaGiribabu Thanks for the contribution, but I'm not exactly sure what you're trying to do here.

• .prior is not a known extension (.py?)
• Your utils import is not in path

Going to close this for now but feel free to ask to reopen, or to open a new PR, if you'd like to comment/add to this PR.

[matthewcarbone]

@SandhyaGiribabu apologies, I see you're referencing #73. I'll reopen this with a few extra comments.

• utils.priors is shorthand for gpax/utils/priors.py, this path should be reflected in your import.
• If this is documentation, I recommend starting off with Markdown format (please use the suffix .md).
• Actually going to keep this closed so as to not trigger unnecessary builds, but feel free to open a new PR with your changes if you'd like.