Store ROIs in scan_info

This MR is about to display ROIs in Flint as part of a scan

Here you can see what is generated in the scan_info.

• It introduce a rois key to store ROIs
• A ROI name is mangled according to the acquisition chain.
• The relationship between ROIs and channels is implicit
• The kind of the ROI is also implicit (from roi name + description keys)

DEMO_SESSION [2]: s = ct(lima_simulator)
   Fri Dec 04 17:28:29 2020: Scan(name=ct, path='not saved')

              roi1_sum  =       0.00000     (      0.00000       /s)  lima_simulator
              roi1_avg  =       0.00000     (      0.00000       /s)  lima_simulator
              roi1_std  =       0.00000     (      0.00000       /s)  lima_simulator
              roi1_min  =       0.00000     (      0.00000       /s)  lima_simulator
              roi1_max  =       0.00000     (      0.00000       /s)  lima_simulator

   Took 0:00:01.619917[s]

DEMO_SESSION [4]: pprint(s.scan_info["rois"])
{'lima_simulator:roi_counters:roi1': {'height': 231,
                                      'kind': 'rect',
                                      'width': 601,
                                      'x': 190,
                                      'y': 113},
 'lima_simulator:roi_counters:roi2': {'a1': -172.11072290449772,
                                      'a2': -300.67835344586587,
                                      'cx': 487.92119883389427,
                                      'cy': 513.0072417312821,
                                      'kind': 'arc',
                                      'r1': 137.91316800547676,
                                      'r2': 198.5387390524788},
 'lima_simulator:roi_profiles:roi3': {'height': 161,
                                      'kind': 'profile',
                                      'mode': 'vertical',
                                      'width': 644,
                                      'x': 202,
                                      'y': 783}}