Plotting variables: `getvars` access & more flexible `setvars`

[sadielbartholomew]

Currently we have a function setvars which can be used to set plotting variables. However, there is no direct user-facing way to access/view those stored plotting variables to see what is defined, which is particularly unhelpful when any future call to setvars will reset any values set back to the defaults, and we may want to retain anything set to non-default values (such as in the test suite, where I have a decorator to store the output plot under a certain name to enable image-based comparison but test often call setvars themselves - though it will be useful for many common user-aligned reasons I can foresee as well).

So, we should make it easy to both:

1. access/get the set plotting variables, notably with a function getvars analogous to setvars to return them as a dictionary; and
2. set new plotting variables with setvars without setting anything not input there to the default i.e. editing any variables defined in the call without changing any other values.

(1) is priority since the latter could be achieved by inputting the former as kwargs to setvars along with any values to change, but that is a bit clunky as the ideal overall solution to (2).