Clean up chemical entity

[NRayya]

Importing "chemical entity" from CHEBI resulted in importing many irrelevant classes Those classes additionally result in heterogeneity of the class molecular entity as some of its children are strucutre based while the others are role based. We will keep the heterogeneity but we need to:

• Keep molecular entity and Has role NMR solvent
• Keep Molecular entity and Has role NMR Reference
• Keep Isotope atoms provided in chebi_terms.txt
• Delete all other instances, leaf classes, and childless parents

[NRayya]

I have deprecated the abovementioned terms with the reason "out of scope" and without suggesting alternatives. d6b3c89

[StroemPhi]

To not get these classes from CHEBI we need to modify the import module to not pull them in instead of deprecating the terms in nmrCV.

[NRayya]

related to #29