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.

I know the arithmetic mean thing is more polite, but I'm going to use
the old version, so that the test still passes as is.
And everyone does it this way anyway...

this approach implicitly solves the anisotropic grid problem