Enterocyte refactor & renaming

[dosumis]

Note - we use the term 'enterocyte' broadly to cover SI and colon, others use it only for SI. Some confusion is arising from this.

1. rename: enterocyte of epithelium of small intestine --> small intestine enterocyte
2. early enterocyte --> early small intestine enterocyte. (synonym immature ...). This is consistent with def.
3. Consider adding synonyms 'late small intestine enterocyte' and 'mature ...' to 'enterocyte of intestinal villus. Villi are only in the SI. Immature (early) are in the crypt. <-- Check this with GCA experts. Also discuss with them which should be the winning name.