Support positional stream args in DynamicMap callback

[jonmmease]

This PR is a step toward supporting #4433 using a DynamicMap transformation. The idea is described in #4433 (comment).

Background

The callback function for a DynamicMap is currently called by passing the kdim values as positional arguments, followed by stream parameter values as keyword arguments. For example:

import holoviews as hv
from holoviews.streams import Stream
XY = Stream.define('XY', x=0,y=0)
xy_stream = XY()

def callback_fn(i_value, y, x):
    ...

dmap = hv.DynamicMap(callback_fn, kdims=["i"], streams=[xy_stream])

Here the i_value argument is the i key dimension that is called positionally (named i_value to emphasize that it's called by position not by name), and the x and y arguments are the stream parameters and they are called as keyword arguments. So here the x and y argument names must match the param names of the xy_stream stream.

This approach is convenient when writing callback functions by hand, when there are only a few stream parameters and the stream parameter names don't conflict. But it's less convenience when building a callback function programmatically that will input many streams with potentially overlapping parameter names. This will be the situation when building the transformation in #4433.

Description

This PR adds a positional_stream_args parameter to the DynamicMap class that, when set to True, will configure the DynamicMap to pass stream values to the callback function positionally. The stream arguments will follow the kdim arguments (if any), the value of each stream argument is a dict containing key-value pairs for each param in the stream, and the order of the stream arguments is determined by the order of the streams in the streams parameter list.

Here's an example of what this looks like:

import holoviews as hv
from holoviews.streams import Stream
XY = Stream.define('XY', x=0,y=0)
xy_stream0 = XY()
xy_stream1 = XY()

def callback_fn(i_value, s0, s1):
    x0, y0 = s0['x'], s0['y']
    x1, y1 = s1['x'], s1['y']
    ...

dmap = hv.DynamicMap(
    callback_fn, kdims=["i"], streams=[xy_stream0, yx_stream1], positional_stream_args=True
)

When positional_stream_args=True there is no possibility of stream parameter name clashes.

Of course, the default is False to match the current behavior.

Does this sound reasonable @jlstevens @philippjfr ?

[philippjfr]

The changes look fine to me, but I'll try to fix the tests before merging.

[jonmmease]

Ok, updated with master and tests are passing. Anything else come to mind for this PR @philippjfr?

[jonmmease]

@philippjfr alright if we merge this today?

[philippjfr]

Yes, looks good to me.

[jonmmease]

Thanks a lot @philippjfr!

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