Develop a repeatable way to create a modelmeta version for the PDP

[corviday]

The PDP uses an older version of the database, which was unaffected by the changes made to accomodate non-gridded streamflow files. Accordingly, working on the PDP requires older versions of the modelmeta and nchelpers packages.

However, sometimes PDP files require changes made to nchelpers after or during the changes made to deal with the database upgrade. This requires building a custom version of nchelpers that includes or rejects commits on an individual basis. This process is slow and frustrating, and I would love to find a way to script it.

[corviday]

I've streamlined the setup and written up a text description of the easiest way I've found on Conflunce, which is likely good enough, though it seems like the process could be scripted.

[corviday]

@jameshiebert suggested an alternate, and less kludgy, solution: maintaining a local branch of modelmeta for the PDP.