Add image-space resolution modelling using Gaussian PSF

[casperdcl]

• [x] add a new fwhm_rm : float argument
  • add SIGMA_RM to resources.py & Cnt
  • in future, this could also support a custom kernel
  • [x] use a faster separable convolution
  • profiling results for 1 iteration: f_prj: 120ms, b_prj: 350ms, filter: 35x2=70ms, scatter: the enemy
  • [x] make sure fwhm_rm=0 doesn't do any filtering
  • [ ] use fwhm_rm=2.5mm by default? Better than nothing for Siddon projectors (see images below)
• [x] update mmrrec
• [x] update mmrsim
• [x] prevent nearly-zero division causing inf

Images

Gaussian FWHM [mm]: Left: 2.5, Right: 4.5

Using 3D filter 969fc2a

Using separable filter 6df9dfd (see diff 969fc2a...6df9dfd)

Profiling (extra time is mostly padding & unpadding Z 127<->128)

Comparison to raw Siddon a19133c (fwhm_rm in [0, 2.5, 4.5])