Allow users to add their own residue templates for building unsupported structures

[gitoliver]

Rene Ranzinger from GlyGen suggested that having the ability to easily add a template for e.g. derivatives like phosphate or an unsupported residue would be great. Even if it doesn't energy minimize we could still provide an initial structure.

Oliver would want something in the HEADER file that unsupported templates were used.

Feature request for the future.