Templating the groups doesn't improve performance that much, but it severely hinders usability. Switch things to be dynamically sized so that different ranked groups are the same type.
Could also now separate headers and implementation... but will probably leave it for now.
Makes tc::Group a direct subclass of Eigen::Matrix. Group should represent a Schlafli matrix.
Added a note that tc::solve assumes the matrix to be for a coxeter group.
In theory we could also implement the "normal" todd-coxeter and allow any Schlafli matrix as input.
