Progressbar: add function to update message in progressbar at runtime
insert custom GSL error handler

This simply throws an error with the error message.
Make GSL error handler report that this is a GSL error
remove __PRETTY_FUNCTION__ from debug output

If anything this only made the output less pretty...
DWI::Shells: new member functions get_counts() & get_bvalues()

Also tweaked INFO messages to use these to list actual counts & b-values