Remove documentation references to old BC types

[valeriabarra]

At the moment, when we build our docs we get the following warning messages:

WARNING: error while formatting arguments for addBC1D: 'NoneType' object has no attribute 'args'
WARNING: error while formatting arguments for addBC1Dlhs: 'NoneType' object has no attribute 'args'
WARNING: error while formatting arguments for addBC1Drhs: 'NoneType' object has no attribute 'args'
WARNING: error while formatting arguments for addBC2D: 'NoneType' object has no attribute 'args'
WARNING: error while formatting arguments for addBC2Dlhs: 'NoneType' object has no attribute 'args'
WARNING: error while formatting arguments for addBC2Drhs: 'NoneType' object has no attribute 'args'
WARNING: error while formatting arguments for addBC3D: 'NoneType' object has no attribute 'args'
WARNING: error while formatting arguments for addBC3Dlhs: 'NoneType' object has no attribute 'args'
WARNING: error while formatting arguments for addBC3Drhs: 'NoneType' object has no attribute 'args'

Although they are non-breaking warnings, they are signaling an issue. I searched in the code and it appears that all these BC types were removed in PR #99 , so the documentation should be updated.

[pritkc]

The issue was resolved with #121 and is being closed upon the successful merger.