Development of a detector model for BPCX
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As described on google documents, a framework to describe the detector in an
X-ray diffraction experiment. This will be implemented in terms of an interface
and a set of implementations starting off with a trivial one.

A detector is defined as a set of sensors with fixed orientation and translation
with respect to each other. A sensor is a an abstract plane with a rectangular
window defined by limits in its two orthogonal directions. This corresponds
to the "Virtual Detector" of the LURE Phase I-III documents.

Here we are transferring code from the bpcx_regression repository that
deals with the detector model, and translating it into C++.