intfbin/msspwglue: exit hack mode: make the spw mapping configurable
intfbin/msphotom: seem to need to specify maxrows now?

With a very large dataset I got a C++ allocation error which I assume
was due to some overlarge allocation request. You'd assume that by
default a reasonable maxrows value would be chosen, though.
intfbin/msspwglue: rewrite the SOURCE table too

I don't think much of anything uses but, but it's the right thing to do. And
concat or something will yell at you if it sees SOURCE entries with invalid
SPECTRAL_WINDOW_ID values.