Use DiracKernel caching instead of doing it by hand in RichardsBorehole.

@WilkAndy may want to review this.

Refs #2364.
Use DiracKernel caching instead of doing it by hand in RichardsPolyLineSink.

@WilkAndy may want to review this.

Refs #2364.
Use the currentPointCachedID() interface.

@WilkAndy may want to review this.

Refs #2364.