basic stats can now be read even in the absence of the main simname.h5
file, using just the cache file.

I'm not sure how much sense this makes, but I am now explicitly dividing
the computed spectrum by dk, such that the integral of E(k)dk is the
total energy.

not tested yet.

for scaling tests, if we don't want to get stuck on I/O things...

it should work for many many particles now.

there were a few issues with read_parameters, which is now a virtual
function in NSVE.

assume each checkpoint file should be at least order 1GB, and then check
how much space each checkpoint will require.
this includes both field and particle data.