make really sure load_mupa has a file to parse.

make really sure load_mupa has a file to parse.
Fixes ticket:121
This includes more filesystem checks to make sure we're reading something
that's a file, and instead of doing fstream.eof() this uses the more
general bool converter for the stream. Which I think is equivalent to
fstream::good() (a more general test than fstream::eof() )