Tests of undulator convergence

Find the test module test_undulator_convergence.py, as well as several other tests for raycing backend, in /tests/raycing.

This is a study of a tapered undulator created from xrt.backends.raycing.sources.SourceFromField. Its magnetic field is shown below.

The output of IntegratedSource.test_convergence() shows convergence of the calculated intensity signaled by various indicators:

Transverse intensity distribution vs number of nodes	Flux distribution vs number of nodes