Support on derived variables

[henry2004y]

Right now the derived quantity plots are not supported. In order to achieve this, I may need:

• A new function getvar(data::Data, var::String) returning the derived variable
• A new plotting function that understands the derived data type

The first one is achieved by a trick I found on discourse, which basically identifies symbols as names to members in a struct.
This test feature is not ideal and will be dropped in later versions. This looks like the Python Calculator in ParaView.
I don't know how ParaView achieve this, but in Python it is pretty easy to parse a string and execute it as some new codes using exec function, as Yuxi did in his interface to the yt library. Julia has equivalent metaprogramming capabilities, but there are many restrictions to apply it in practice so it is generally adviced to avoid evaluating expressions inside functions during runtime.
Another option is to create functions for derived variables. This is actually pretty good both in terms of performance and conciseness. The idea is to create a dictionary of derived variable names as keys and anonymous functions as values, and if the input string is found in the dictionary, call the corresponding function to obtain the value.

This has been successfully tested in my new scripts for processing Vlasiator outputs, and can be directly ported here for BATSRUS.

[henry2004y]

A more complete variable list can be found from the IDL script. A similar implementation can be found at vslvvariables.jl