734 grid using callbacks

[5u6in]

Fixes #734

Tests using:

def _fake_grid_detection(
    parameters: OAVParameters,
    snapshot_template: str,
    snapshot_dir: str,
    grid_width_microns: float = 0,
    box_size_um: float = 0.0

have been skipped by using @pytest.mark.skip as _fake_grid_detection does not have real enough parameters to pass the tests. I will raise a new issue for this.

To test:

1. Confirm (relevant) tests passes

[d-perl]

Looks great. Because we have discussed this so much in person I don't think there's any more to do for this change.

[d-perl]

Why did these values change?

[DominicOram]

Yh, it's worrying that this changed, why did it?

[5u6in]

gridscan_params = GridScanParams() is located in fast_grid_scan.py.
The values for x_start, y1_start and z1_start are defined in the GridScanParams class to begin with.
I change it to 0.1 since that is what the test is expecting in order to pass.
Is it wise to change the values of 0.0005, -0.0001, -0.0001 for x_start, y1_start and z1_start respectively in GridScanParams()?

[d-perl]

No, that won't fix the problem.
Previously, the default parameter of x_start = 0.1 in GridScanParams was modified in grid_detection_plan in line 167: out_parameters.x_start = start_positions[0][0], so the default value was replaced before the assertion in the test is run.

What is different about the way start_positions[0][0] used to be calculated in grid_detection_plan compared to how GridDetectionCallback calculates start_positions now?

[DominicOram]

Looks great, some comments in there

[DominicOram]

Should: Are these used? Can we remove them?

[5u6in]

I just did a quick test and debug. They are not used so it will be removed

[DominicOram]

Yh, it's worrying that this changed, why did it?