.value was deprecated, so I now use [...], which should work for old
enough versions (they recommend [()] instead).

also, fix typo in __main__.py

NSVEparticles test works.

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.