Check SLMImage exposure input values

[hinderling]

Currently even negative values are allowed,

event = MDAEvent(slm_image=SLMImage(data=True,exposure=-999))
mmc.mda.run([event])
--> runs

where as this is checked for camera exposure:

event = MDAEvent(exposure=-99)
--> validation error for MDAEvent

Not sure if this needs to be checked, because negative values are allowed on device level (Polygon1000, genericSLM):

mmc.setSLMExposure(slm_dev,-999)
print(mmc.getSLMExposure(slm_dev))
--> -999.0

[tlambert03]

good catch! pydantic makes this easy, let's copy this pattern:

useq-schema/src/useq/_mda_event.py

Line 192 in 4b42d1e

exposure: Optional[float] = Field(default=None, gt=0.0)