Get a point's coordinates along a line.

Please consider this patch for an eventual merge. Thank you.

Get the coordinates of a point along a line defined by p1 and p2, at a specified offset relative to p2.

This has been discussed here :

https://discourse.slicer.org/t/rfc-should-this-method-be-generally-available/28332

Thanks to Andras Lasso (PerkLab) for sanitizing the code.