Inclusion of custom functional groups.

[lipelopesoliveira]

Motivation

The current version of the code does not allow a simple way to include custom functional groups, as noted in #84

What should be done?

Modify the BuildingBlock class to allow custom functional groups.

Steps for the realization of the task:

What will be obtained?

A new version of the code that allows the use of custom functional groups.

Is there any deadline?

Any other thing?

Documentation and examples should be created.