Get rid of too many descriptor caches

[mojaie]

@cache macro enables reuse of intermediate results of molecular descriptor calculation for fast computation, but I don't know whether it is really effective.

Tentatively I wll remove @cache from molecular descriptor functions except for computationally expensive and frequently used functions listed below.

• mincyclenodes (for SSSR detection)
• coords2d

[mojaie]

Here is a benchmark result of execution times of descriptor functions per 20 molecules, n=100, median. At least, SSSR caching seems to be very effective.

[mojaie]

Execution times per molecule of descriptor functions , (20 molecules dataset, n=100, median). Caching valence arrays (lonepair, valence and apparentvalence) may work.

[mojaie]

0d84cb7