Option to force "number" rather than "integer" in Signal

[cjtitus]

Summary

There are many signals where the dtype is known a-priori to be floats. However, Ophyd always looks at an example value when describe is called, and shoves that into a data_type function.

If the first value happens to be an integer, the describe method then returns integer, and all further data will be cast to an integer, even if everything else is floats. I think the true data is saved, somehow, but have never really managed to get it back out of Tiled.

There should be a way to tell a signal to treat numeric dtypes as number and not integer, so that this can be set properly on a signal-by-signal basis.