Method to create a pyFAI detector object

[takluyver]

In an MID example notebook, EXtra-geom geometry was used in pyFAI like this:

import pyFAI.azimuthalIntegrator
import pyFAI.detectors

agipd = pyFAI.detectors.Detector(200e-6,200e-6)
agipd.aliases = ["AGIPD1M"]
agipd.shape = (16*512,128)
agipd.mask = np.zeros((16*512,128))
agipd.IS_CONTIGUOUS = False
agipd.IS_FLAT = True
agipd.set_pixel_corners(geom.to_distortion_array())

px = 570
py = 685
ai = pyFAI.azimuthalIntegrator.AzimuthalIntegrator(detector = agipd)
ai.setFit2D(7800,px,py)
ai.wavelength = 1.38e-10

The first half - making a pyFAI detector object - looks like it could easily be wrapped up in a method in EXtra-geom.

cc @dallanto @jo-moeller