Rename inference mask in "InferenceMaskMap"

Following discussions in NIDASH call on September 22nd ([Minutes](https://docs.google.com/document/d/1VmHvlXFYuFwaZwdou0zhSdtoj1vQjxAGICyeoAKvl8Q/edit?usp=sharing)) and at #157, this pull request rename the Mask entity used for small volume correction in `InferenceMaskMap`.