all unmeasured pixels treated as unconstrained

[a4894z]

currently we use a fixed unmeasured pixel binary mask to define regions where we do not enforce the measurement constraint. this fix allows us to treat all unmeasured pixels as unconstrained.

this was an issue for us for recent beamtime where some measurements had saturated low Q pixels, and by enforcing these saturated pixels, the reconstruction would have crappy artifacts.

we tried to zero these pixels out, and enforced those zeroed out low Q regions, but this would cause another set of crappy artifacts.

simply leaving all unmeasured pixels unconstrained when performing exitwave updates solved this problem for us, and here's the code I used to make this happen.

[mdw771]

• Renamed the option to exclude_measured_pixels_below: it sets the gradients of pixels corresponding to measurements <= the set value to 0.
• The option is now in ReconstructorOptions.
• Extended valid pixel mask and this dynamic thresholding mask to all reconstructor, not just LSQML.