Un-deprecate CMAP registration

test/test_plot.py::test_plots
  /builds/ift/nifty/nifty8/plot.py:266: PendingDeprecationWarning: The register_cmap function will be deprecated in a future version. Use ``matplotlib.colormaps.register(name)`` instead.

See https://gitlab.mpcdf.mpg.de/ift/nifty/-/jobs/1948616#L528

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