Nested JSON structure for lines and fiducials

[mariakakis]

Right now, the code only accommodates up to 3 lines and 1 fiducial. I can imagine cases where people would want to use more than one fiducial, and I've seen RDTs with more than 3 lines (https://www.medicalogy.com/images/product/thumb_rapid-test-malaria-pf-pv-whole-blood.png). Ignore for now, but might be nice for later

[hungngo97]

You mean like an array of Fiducial object and Line object right (with their position and width) ? Agreed.

[mariakakis]

Pretty much, but this is very low priority since it could break lots of things if we aren't careful