adding different distance relationship for collimator response modelling for SPECT

STIR currently uses sigma = a d + b, while many other people use sigma = sqrt(sigma_intr^2 + (d a)^2). It'd be nice to support the latter as well. Obviously, people will have to check which one fits their scanner/collimator best.

Necessary code modifications seem quite small. The actual code to generate sigma is
https://github.com/UCL/STIR/blob/e2f0bcb74af6bbf067785e07f58a8430faf3c64d/src/recon_buildblock/SPECTUB_Tools.cxx#L540 (note: do_fb is for fan-beam collimators)

• we'd need an extra field COL.do_parallel_with_sqrt. could reuse the A and B fields to make things confusing, or add 2 new ones. I don't mind which really as this is internal code.
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• we'd need to add members/methods and keywords for ProjMatrixByBinSPECTUB, e.g. at
  https://github.com/UCL/STIR/blob/e2f0bcb74af6bbf067785e07f58a8430faf3c64d/src/recon_buildblock/ProjMatrixByBinSPECTUB.cxx#L103-L104

Start at src/recon_buildblock/SPECTUB_Tools.cxx around line 540, where sigma is generated, and inspect the do_fb handling. Then read ProjMatrixByBinSPECTUB.cxx around lines 103-104 for the related members, methods, and keywords. Done means the alternate distance relationship can be configured and used for SPECT collimator response modelling.