Change modality parameter pixelsize_nm to take float

Is your feature request related to a problem? Please describe.
User can change the pixel size and resolution of the modality they are using to match their experimental set-up, but the precision of the pixel size is limited to 1 nm, whereas in practise it should be ~0.01 nm. When comparing the size of structures from an imaging experiment to a vLab similation, the precision of the pixel size might cause avoidable (?) discrepencies. For example, I would like to compare nanocage size with those generated from simulations of nanocage labelling at varying efficiencies.

Describe the solution you'd like
Would prefer this variable to be a float, instead of an integer.

Describe alternatives you've considered
None

Find where the modality parameter pixelsize_nm is defined and how its value is parsed or validated. Update the parameter handling so decimal nanometre values are accepted, then run the relevant project checks to confirm existing integer values still work and fractional values are preserved.