egs++ shared library source

[blakewalters]

Is your feature request related to a problem? Please describe.
It would be nice to have the ability to use an egs++ simulation as a shared library source, similar to the way we can currently use a BEAMnrc simulation as a source.

Describe the solution you'd like
Inputs would be similar to a BEAMnrc shared library source, with the user supplying the name of the egs++ application (compiled as a shared library), its input file, and the pegs data file (if not running pegsless).

The source simulation must be set up to use a phase space scoring ausgab object, but, instead of writing these particles to a file, they would be stored in a source container that is then accessed by the 2nd-stage simulation using the source.