Remove requirement for the user to define groups

[ianhinder]

In the current version of Kranc, the user needs to group the evolved variables into named "groups" which become Cactus groups. Most users of Kranc are using tensorial variables, and Cactus requires (for symmetries to work correctly) that each tensor is in its own group. As a result, there is no reason for the user to have to specify groups, as this could be automatically determined from the tensors. Removing the need for users to specify groups would simplify the interface and make it easier to use.

We would still need group information for variables from other thorns.