Consecutive bond descriptors

[gervasiozaldivar]

The grammar implementation allows for a BigSMILES string to have two consecutive bond descriptors, which should be forbidden. We must add an exception when this happens.
Example:
obj = gbigsmiles.BigSmiles.make("NN{[<] [>][<]CC[>][<] [>]}|gauss(1500, 50)|Br.|100|")
To do list:

• create exception for two consecutive bond descriptors