Disambiguate rake fixtures tasks

[mark-dce]

• [x] Add :models namespace to :fixtures task in tufts_models
• [x] Remove :fixtures task from tufts-image-library
• [x] Add new fixtures from comments in #78

[mark-dce]

• Add namespace: https://github.com/curationexperts/tufts_models/commit/c1bf1aae4facef78ca144135411f2357553cad4e
• Remove task: https://github.com/curationexperts/mira/commit/94e5bfa5df56cfbade4d66ad7056dd787acf12bb
• Add fixtures: https://github.com/curationexperts/tufts_models/commit/c1bf1aae4facef78ca144135411f2357553cad4e