Remove labs

Remove `nipy.labs` module. As a consequence, the `libcstat` C-library is removed too, hence NIPY no longer has a dependence on Lapack. 

Some modules previously found in `nipy.labs` are moved as follows:

`nipy.labs.viz` --> `nipy.algorithms.viz`
`nipy.labs.datasets` --> `nipy.algorithms.viz.datasets`
`nipy.labs.mask` --> `nipy.algorithms.mask`
`nipy.labs.spatial_models` --> `nipy.algorithms.spatial_models`
`nipy.labs.statistical_mapping` --> `nipy.algorithms.statistics.spatial_stat`
`nipy.labs.utils.simul_multisubject_fmri_dataset` --> `nipy.algorithms.utils.simul_multisubject_fmri_dataset`
`nipy.labs.utils.reproducibility_measure` --> `nipy.algorithms.utils.reproducibility_measure`

The other modules, which are Lapack-dependent, have migrated to another project (https://github.com/nipy/nipy-labs) and are available as before under `nipy.labs` if that project is installed. If not, importing `nipy.labs` results in a warning message.