Merge pull request #695 from samaloney/doc-quantity-gwcs

Add example creating GWCS from quantities and times.

changelog/695.doc.rst

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+Added a gallery example  (:ref:`sphx_glr_generated_gallery_creating_a_gwcs_from_quantities.py`) showcasing how to create a GWCS from quantities.

examples/creating_a_gwcs_from_quantities.py

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+"""
+===============================================
+How to create an GWCS from quantities and times
+===============================================
+
+This example shows how to create a GWCS from astropy quantities.
+"""
+import numpy as np
+from matplotlib import pyplot as plt
+
+import astropy.units as u
+from astropy.time import Time
+
+from ndcube import NDCube
+from ndcube.extra_coords import QuantityTableCoordinate, TimeTableCoordinate
+
+##############################################################################
+# We aim to create coordinates that are focused around time and energies using astropy quantities.
+
+energy = np.arange(10) * u.keV
+time = Time('2020-01-01 00:00:00') + np.arange(9)*u.s
+
+##############################################################################
+# Then, we need to turn these into lookup tables using
+# `~ndcube.extra_coords.table_coord.QuantityTableCoordinate` and
+# `~ndcube.extra_coords.table_coord.TimeTableCoordinate` to create table coordinates.
+
+energy_coord = QuantityTableCoordinate(energy, names='energy', physical_types='em.energy')
+print(energy_coord)
+
+time_coord = TimeTableCoordinate(time, names='time', physical_types='time')
+print(time_coord)
+
+##############################################################################
+# Now we need to combine table coordinates created above and extract the ``.wcs`` from the result.
+
+wcs = (time_coord & energy_coord).wcs
+print(wcs)
+
+##############################################################################
+# Now, we have all the pieces required to construct a `~ndcube.NDCube` with this data and the GWCS we just created.
+
+data = np.random.rand(len(time), len(energy))
+cube = NDCube(data=data, wcs=wcs)
+print(cube)
+
+##############################################################################
+# Finally, we will plot the cube.
+
+cube.plot()
+
+plt.show()