pyCalculations/derivatives, VlsvWriter updates

[alhom]

-VlsvWriter update for VisIt vlsv plugin compatibility -VlsvReader utilities for reading, crude down- and upsampling, read_fg_var_as_volumetric -pyCalculations/derivatives.py: Poynting flux and div thereof, curvature and ballooning criteria (w/ Jonas)

[alhom]

Yes, it's a bit awkward to have these separate (and frankly, I wasn't really sure what would be the best approach to begin with - so this ballooning stuff got collated to derivatives.py), but there were a couple of reasons to have this bunch on its own for the time being:

• reliance on fsgrid variables (and re-centering them!), which have been somewhat fiddly to work with
• the expectation that the outputs are to be written back to a sidecar, instead of being only used as an expression in plotting (so, not relying on plot_helpers top-level variables, or 2D plane choices for output formatting, either)

I could see some (or even most) of these as reducers, even - at least in principle, the operators there could be extended with differential ops (though, calling those datareducers is a bit much!). Thoughts?

[alhom]

This is still supplanted by an incoming PR to chop this up a bit.