Implement atomic gradients for non-scalar CVCs

[giacomofiorin]

Currently they are either calculated on the fly (distanceDir, distanceVec) or stored in a different data structure (orientation -> cvm::rotation object). There can be two solutions to this:

• add additional 3-vectors in each cvm::atom object (problem: increases memory usage for all variables);
• store additional vectors of 3-vectors as needed in cvm::atom_group (problems: breaks data contiguity).

[giacomofiorin]

Included by issue #61