Warn on gotocen

Closes #1880 (closed)

This PR display a warning in case of discrepancy between the display in Flint and the plotselect.

Notice that the selected counter have actually nothing to do with Flint. It only depend on plotselect. If you select something else in Flint in between, it is not used by goto_cen.

Here is the kind of warning you can have:

DEMO_SESSION [8]: flint(); umv(slit_vertical_gap,.1);ascan(slit_vertical_offset,-1,1,30,.1,beamviewer)
WARNING 2023-02-08 14:24:52,608 flint: Flint starting...
Current Flint PID:  18869

Moving slit_vertical_gap from 0.1000 to 0.1000

     slit_vertical_gap      slit_top          slit_bottom
user             0.100              1.050             -0.950
dial             0.100              1.050             -0.950

DEMO_SESSION [9]:
DEMO_SESSION [9]:
DEMO_SESSION [9]: plotselect("beamviewer:roi_counters:roi1_sum")

Plotted counter(s) last selected with plotselect (could be different from the current display):
- beamviewer:roi_counters:roi1_sum

DEMO_SESSION [10]: goto_cen()
Moving slit_vertical_offset from 1.0000 to -0.2331
Moving slit_top from 1.0500 to -0.1831
Moving slit_bottom from -0.9500 to 0.2831

DEMO_SESSION [11]: goto_cen()
WARNING: The used counter name 'beamviewer:roi_counters:roi1_sum' is not the only one displayed in Flint
Moving slit_vertical_offset from -0.2330 to -0.2331

DEMO_SESSION [12]: goto_cen()
WARNING: The used counter name 'beamviewer:roi_counters:roi1_sum' is not actually displayed in Flint
Moving slit_vertical_offset from -0.2330 to -0.2331