Multi valued parameters

[MerlinSmiles]

I've been running into issues with multi valued parameters.

i.e. in the magnet driver, I want a fld parameter that returns and sets x, y, z values, reading the values works just fine with the Anything validator, but setting failes.

For reference, something like this:

self.add_parameter('fld',
                   names=['B'+ax.lower() for ax in self.axes],
                   get_cmd=partial(self._get_fld, self.axes, 'FLD'),
                   set_cmd=partial(self._ramp_to_setpoint, self.axes, 'FSET'),
                   units=['T' for ax in self.axes],
                   vals=Numbers())

There are at least two issues here:

1. in the parameter.py file, lines 146+:

        if names is not None:
...
        elif name is not None:
...
        else:
            raise ValueError('either name or names is required')

The multi valued parameters no longer have a name, and a _vals attribute anymore.

If I do

        if name is not None:
...
        if names is not None:
...
        if (name is None) and (names is None):

It kindof works, but at least the docstring and units is broken (and I have to use the Anything validator)
2. The validators only work on single-valued inputs, not on multi-values.

[Rubenknex]

I also encountered a command in which I need to pass two arguments. It would be nice if this was included in the functionality of the Parameter class.

[giulioungaretti]

@MerlinSmiles let's look at this when you are back here, it seems like that can be fixed creating a custom parameter instead of the standardparameter that add_parameter uses by default.

[jenshnielsen]

related to #378, #166 and #498

[jenshnielsen]

I believe this was solved along with #166