Create Merge meshtal capable of handling h5m files containing advanced tally data

[makeclean]

Need to create a script that can combine the data stored in h5m mesh tallies. Should have the same functionality as the mesh_meshtal version when that is decided. Perhaps consider writing this in Python using the PyTaps interface, perhaps @zwelchWI should speak with either @erelson or @elliottbiondo due to their experiences with PyTaps.

[makeclean]

Definition of possible merge operations

• Average across files
• Sum all within file
• Sum within file and average across files
• Sum some within file
• Sum some within file and average across files
• May want alternative ways of estimating the error other than weighted average, e.g. estimation of error by determining std dev around mean.

Will need error checking to ensure merging the same meshes in each case

WIll also need to consider how little/much memory these operations will take when we perform these operations with large meshes and thus plan the algorithms accordingly.