Support for user specified projection in vector plot function

The vector plot function by default plotted data on the same projection as the source data.

For the purposes of comparing RCMs and GCMs it is desirable to be able to manually specify the projection, so that all models are compared on the same projection.

I have implemented this functionality in a branch, as well as a couple of other bug fixes to the function. However, when I tried to push my branch to the repo I got a message saying I didn't have permission to do so..

Start by locating the vector plot function and reviewing the reporter's existing branch, then check how it currently derives the source projection. Confirm the desired behavior by comparing RCM and GCM plots on a manually selected projection; done means the function accepts and uses that projection, with the branch's additional fixes separately understood.

Written by the indexing model from the issue text.