Panda grid scans don't correctly check for gridscan_valid

[olliesilvester]

PandA grid scans tried to kickoff a gridscan while the BL03I-MO-SGON-01:PGS:SCAN_INVALID was set to invalid (0). IOC logs show that panda grid scan parameters had been updated, so looking at the code, it is unclear why this PV wasn't being checked properly in wait_for_gridscan_valid. See graylog error: https://graylog.diamond.ac.uk/messages/graylog-daq-mx-hyperion_21/3c7fd880-702f-11ef-bc16-26706383e36b

One possible cause:
The invalid PV takes up to 0.1s to update after one of the grid scan parameters change (I think). Hyperion may be doing wait_for_gridscan_valid before the PV has properly updated. Within the time resolution of the IOC logs, this looks possible (but resolution too small to say for sure).
If that is the issue, not sure why we wouldn't have seen it for zebra FGS.

Acceptance Criteria

• Issue is fixed, potentially by just adding a short sleep in wait_for_gridscan_valid