ENH plot:  plotting without Colorspace

- 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()
TEST plot:  close() plots
RF ndvar.get_data():  use np.transpose()
ENH ndvar:  better time indexing accuracy through rounding
ENH as_factor():  convert any object that has .as_factor() method
ENH data-obj:  assub() to interpret sub argument
FIX/TEST plot.uv
ENH Celltable:  sort data
API test.resample():  _resample -> resample(); iterate only over Y
ENH Celltable:  cat arg to only load certain categories;  testnd.ttest use that
FIX plot:  .set_vlim() methods
FIX plot:  .set_cmap() methods
FIX dim:  Sensor.neighbors()
ENH data-obj sorting:  `order` kwarg
ENH Celltable:  sort cases (&TEST)
ENH: permutation cluster test for more multidimensional ndvars