update doc section about get_data
https://bliss.gitlab-pages.esrf.fr/bliss/master/scan_engine_scan.html#scan-data
seems to be outdated. It does not mention that also counters and motors can be used as keys in scan.get_data
https://bliss.gitlab-pages.esrf.fr/bliss/master/scan_engine_scan.html#scan-data
seems to be outdated. It does not mention that also counters and motors can be used as keys in scan.get_data