Add box-drawing facility to measurePSF

[raacampbell]

The idea is to make an interactive version of the frameSize optional input argument from the old function.

• [x] Allow the user to draw a box around a bead and do a PSF fit to this.
• [x] All panels should change their axes to centre on this bead.
• [x] Cross-hairs centre on new bead.
• [x] Tidy up mentions of frameSize (8eae55d555cc82e803dae3db06a4efba5b0da5b9)

[raacampbell]

Zooming: ccad4d780be21953b4f93a4f97b9d3d6c5e5b2fb

[raacampbell]

Still need to test this with real data not just the simulated PSF. But seems good so far.

[raacampbell]

Works