cf-plot (& dependencies) inclusion in `cf.environment()`

[sadielbartholomew]

cf-python has an environment function which reports the versions, and optionally paths to, itself and its dependencies, especially useful for user support where we ask users to share the output they see from that so we can quickly learn about the exact versions they are using for anything relevant. It would be good to have a cf-plot equivalent so that we can use it in cf-plot itself (users may not be using cf-python explicitly, so have to import cf to access it in code importing only cf-plot) and notably get information about extra cf-plot dependencies such as Cartopy etc.

I propose:
1. We create a cf-plot equivalent, cfp.environment(), which wraps cf-python to report on anything cf-python needs, since cf-python is a cf-plot dependency, but adds any further dependencies cf-plot has to the list. This separates concerns nicely.
2. cf.environment actually reports the version of cf-plot and some of its dependencies (notably matplotlib) despite these not being relevant to cf-python itself, so I also propose we move anything relating to cf-plot such as cf-plot and matplotlib out of that and into cfp.environment().

[Proposal changed - see next comment bold text. Ultimately, we want to provide a way to get crucial version and dependency information for both cf-python separately and cf-python plus cf-plot.]

[sadielbartholomew]

I have actually changed my mind about this, for the following reason mostly: if we have a user support request (one of the most common use cases for cf.environment() to be called), then having a cfp.environment() would require the user to report two outputs separately, whereas at present it is just the one from cf.environment which is only missing one library (Cartopy) to include all dependencies in the report.

Therefore I think it is better to add Cartopy to cf.environment instead, so that all version info for cf-plot is covered by that, perhaps with a cfplot_deps or similarly-named keyword argument (default True unless that is controversial) which would toggle whether or not to report on the version and version of dependencies to cf-plot (just Cartopy at the moment on top of cf-python) as well as those for cf-python, else leave those out (to avoid clutter). And that would sort it all.

I'll change the name of this Issue accordingly and transfer over to cf-python.

[sadielbartholomew]

@davidhassell: not an urgent issue, but is one that will take about 5 minutes to make a PR for and close, so let me know what you think when you have time. In short I suggest we add Cartopy to cf.environment and perhaps a toggle to include or not cf-plot and cartopy from the output as per my change of mind comment above.