opt_einsum VS xrt

opt_einsum: for speed

I am working on simulations in the x-ray range, so fundamentals like propagators are the same, but approximations of optical elements and interactions are different. The x-rays are emitted by a synchrotron radiation source, i.e. an electron storage ring. The statistical properties of the wavefronts are determined by the phase space of the electrons in the storage ring, which is usually modeled by a 5D Gaussian distribution (space + momentum + energy). People are interested in the coherence properties of the emitted ensemble of wavefronts, so the simulation consists are drawing random parameters from the 5D Gaussian, calculating the wavefront emitted and propagating it to the points of interest. There, the wavefronts are collected and their coherence properties are evaluated. I use the coherence module of XRT for that: https://github.com/kklmn/xrt/blob/master/xrt/backends/raycing/coherence.py