ignore output files of Devel::NYTProf
simplify and optimize PDL::NDBin::Iterator

Profiling indicated that a very significant amount of time is spent in
Iterator.pm, and particularly in next1() and next(). This commit both
simplifies and optimizes the iteration: there is now a single method
advance() which has been optimized somewhat. Some preliminary testing
indicated a speed-up with a factor of three. The performance bottleneck
in advance() may very well be the reason that PDL::NDBin performs so
poorly on very large bin numbers. (Note that a lot of time is spent on
looking for the next active variable, so advance() is bound to waste a
lot of time.)
add more tests on number of bins when not supplied
skip files generated by Devel::Cover and Devel::NYTProf from dist
exclude local test data from dist
v0.007

 - simplified and optimized PDL::NDBin::Iterator somewhat, in an attempt to
   address the poor performance with large numbers of bins (Edward Baudrez)
 - added more tests in an attempt to catch a strange test failure that I cannot
   reproduce (Edward Baudrez)