Pre-processing: Adding CTF based single-distance phase-retrieval

Some of the ESRF beamlines (e.g. ID16A) often use different filters other than the Paganin filter for phase-retrieval. Here we ask for the inclusion of support for the contrast transfer function (CTF) single-distance phase-retrieval.

A comprehensive description of the CTF and it's derivation / usage can be found here: https://doi.org/10.1364/OE.26.011110 ( equation (8) for single distance; Equation (16) for multiple distances).