Clarify names for OAT analysis

[abachma2]

The current names for some of the cases in the one-at-a-time (OAT) sensitivity analysis in the haleu project are not very clear. Currently, the directory and variable names for variations in the advanced reactor build shares are just the names of the reactor (e.g. mmr). This can be confusing now that OAT includes variations in the discharge burnup of the Xe-100 and MMR.

For this issue to be closed, the variable and directory names for variations in the advanced reactor build shares should be updated to reactornames_share, e.g. mmr_share to better distinguish what is being changed with this variable and to better distinguish it from variations in the Xe-100 or MMR burnup.