As all other sibling gravity fields resource folders for test purposes, only ONE file in
this directory must match the default pattern for the format as defined in the class
org.orekit.forces.gravity.potential.GravityFieldFactory. All other files names must NOT
match the default pattern.

This ensures that when we specify only the directory name in the data roots (i.e.
potential/egm-format, or potential/grgs-format, or potential/icgem-format, or
potential/shm-format), then we know for sure which file will be selected by the default
configuration.

Loading the other files (the ones which do not match the default pattern) is done in test
files by explicitly setting a reader with the specific non-default name and adding the
reader to the factory by calling GravityFieldFactory.addPotentialCoefficientsReader.
