Nitro groups should have two acceptors.

Discussion from: http://goo.gl/OqUAHD

In this commit I did the following:

- Removed (my) previous incorrect Acceptor setting on Nitro group (was doing
that on the N atom)
- Modified ChalcAcceptor in BaseFeatures.fdef definition to also match
Nitro Os
- Added a test for the above in testChemicalFeatures.py.

Some questions I have about this are:

- Was there a specific reason why the BaseFeatures.fdef file did not have a
test?
- A silly one, no doubt, but what does "ChalcAcceptor" stand for?
- In Contrib/M_Kossner/BaseFeatures_DIP2_NoMicrospecies.fdef the definition
of ChalcAcceptor was:
```
    AtomType ChalcAcceptor [O;H0;!$(O=N-*);!$([OD2]([#6])[#6])]
```

Doesn't this rule out the O in COC as an acceptor?