Improve cylinder geometry

Ideally we would like the option of treating a hollow sheath, rather than a full cylinder. That requires changing the radiative calculations. For IC it's messy because of the RT calculations (but as a first step we can just keep the filled cylinder?). For cyclosycnrotron we would need to do something like the IC RT corrections: take a full RT code, generate a table of sheath spectra for known parameters (viewing angle and sheath thickness I suppose?), create a table of radiative transfer corrections, and just interpolate that table afterwards.

The issue names no files, tests, or entry points; begin by locating the cylinder-geometry and radiative-transfer calculations in the C++ model. Clarify whether the first deliverable is a filled-cylinder IC change or the larger synchrotron sheath lookup and interpolation work, then define validation before implementation.

Written by the indexing model from the issue text.