ENH plot:  flexible cmap and vlim

- replace Colorspace class with info dict entries
- add "meas" info entry
- set_cmap() and set_vlim() methods for plots
ENH dimensions: add dim.unit attr, use for naming when Xax=".dim"
STY testnd:

- cluster_dist -> ClusterDist
- remove pmax parameter from ClusterDist
API:  create eelbrain.data package

- RM duplicate:  remove cell_labels(), only retain cellname()