Add egs++ volume calculation

[rtownson]

Add a Monte Carlo volume calculation to egs++, originally designed by @ftessier. It can be turned on using :start volume calculation: in any input file. The recursive algorithm builds an octree with higher resolution in geometries with more regions. A user specified box is used as the boundary for sampling.

For example:

:start volume calculation:
    passes = the number of recursion passes to create the octree
    samples = the number samples to perform per node per pass
    box min = minX minY minZ
    box max = maxX maxY maxZ
    labels = labels you have added to your geometry using `set label`
:stop volume calculation:

For the volume calculation, the sampling box is divided into progressively smaller voxels as the geometry is analyzed on each pass. The algorithm follows the creation of an octree. Areas with a greater density of geometry regions end up with a higher density of voxels and thus are sampled more often in the volume calculation.

As the volume calculation progresses, the number of samplings in each octree node is also adjusted. On each pass, regions with a higher uncertainty on the volume are sampled more often, while regions with lower uncertainty are sampled less often.

I'm open to suggestions on this implementation! Is is bad practice to add the Volume_Node and Volume_Info classes to egs_base_geometry.h? Maybe these should go in a separate file. Is there additional or alternative functionality we would like?

[rtownson]

@ftessier has an update and bug fix pending for this PR.

[ftessier]

Indeed, we need to clean this up further in terms of memory management and uncertainty tracking. I will merge this into develop after the release, to be merged in master in 2018.

[rtownson]

Redesign suggestion:

The volume calculation should be restructured as a class and should contain methods for applications to query the geometry volumes, instead of just printing to the screen.

The inputs should not be available to any application, rather the application should parse and handle the volume inputs. We should include an egs_volume application for this. This is to avoid the user submitting an egs_chamber run to a cluster with the volume calculation accidentally turned on.

Parallel run capability should then be added to the egs_volume application.

[mainegra]

@ftessier and @rtownson , are we still deciding what the best course of action is? Could we not merge egs_volume as it is and then continue its improvement?
As it currently stands users would benefit tremendously from having such a "little" app at their disposal for geometries whith very complex regions.

[ftessier]

I propose to include this via a standalone application, and not via the egs++ geometry. Usage is then clear for the user, and development is freed of the concerns of integration in the geometry library (it could be integrated back as a geometry method later on). I will convert this PR back to a standalone egs_volume application, and merge it into develop just after the Release 2021. The idea is that any egs++ geometry input file with a volume calculation input block.could be run with egs_volume to calculate the region volumes.