improve handling of dependencies [installation / conda]

[raphaelquast]

• improve startup time by loading modules only when they are required
  (this is already done but it can be improved)

• split package into eomaps_base and eomaps

  would be nice to be able to install a "minimal version" and a "full version" of eomaps (similar to matplotlib_base and matplotlib) ... I don't really know how to do this properly... any help is highly appreciated!)

  • eomaps_base: only the most basic needs (e.g. matplotlib, cartopy, numpy etc.)
  • eomaps: eomaps_base and all optional dependencies ( e.g. pyqt, datashader, xarray, rasterio etc.)