Write utility functions to easily convert input files from mpm-benchmarks

[chahak13]

cb-geo/mpm has multiple benchmarks that we would like to emulate here too but our input format is slightly different for files like entity_sets.json, particles.txt etc from there. Write some utility functions that can easily convert these input files from https://github.com/cb-geo/mpm-benchmarks into input files used by diffmpm.