BUG: Change declaration of SynAxis components such that describe() works properly.

[marrable]

What is this PR about?

When using SynAxis for simulated testing, calling .describe() on an instance returns a descriptor for integers rather than floats.

Example code that demonstrates the issue

from ophyd.sim import SynAxis

if __name__ == "__main__":
    sim_motor = SynAxis(name="motor1", value=3.14)
    print(sim_motor.describe())

Which produces:

OrderedDict([('motor1', {'source': 'SIM:motor1', 'dtype': 'integer', 'shape': [], 'precision': 3}), ('motor1_setpoint', {'source': 'SIM:motor1_setpoint', 'dtype': 'integer', 'shape': [], 'precision': 3})])

What is the fix?

Declaring the components for SynAxis with a value of 0.0 rather than 0. Running demonstration code above with the fix in this PR produces:

OrderedDict([('motor1', {'source': 'SIM:motor1', 'dtype': 'number', 'shape': [], 'precision': 3}), ('motor1_setpoint', {'source': 'SIM:motor1_setpoint', 'dtype': 'number', 'shape': [], 'precision': 3})])

How did I encounter this issue?

While using the bluesky LiveTable callback, format errors were occurring where string format was being called for float values on integer format strings.

[tacaswell]

Thank you @marrable

It would not hurt if we also added an internal cast to float, but this way at least the default value is not a trap.