Note: This functionality is not yet feature complete and
only basic usage has been tested. The cdn.hdf file will still
change.

Conflicts:
	vgen/convol.f90

This is to make it usable for situations where ufft is not
the step function. Christian has such situations.

Also corrected the documentation in convol.

Now the attribute minDistance is read in from the input file into
the variable input%minDistance. By default this variable is 0.0 but
it can be set to finite values in the inp.xml file. It is supposed
to be an additional optional convergence criterion to stop calculations
when they are converged.