RichardsBorehole: added ability to specify one point only, along with a length and direction.  Tested.  Fixes #3561
Merge pull request #3562 from WilkAndy/one_dirac_3561

RichardsBorehole: added ability to specify one point only, along with a ...
Merge commit 'c7f4157f2afb300ad6217a7617b5f8479912cda2'