visualize.FeatureSpace needs setters for raster data, spatial ref. info

[arthur-e]

Currently, visualize.FeatureSpace() expects to be given a path to a raster file, which it then reads in, setting its internal data structure and attributes. Need to provide setters so that:

• An existing gdal.Dataset or np.ndarray (with gt and wkt also provided) can be used to initialize a FeatureSpace instance;
• Either an MNF-transformed or non-transformed raster/ array can be provided and plot_spectral_profile() still works;