Resolve dependent functions in Dynamic operations

[philippjfr]

This makes it possible to provide a function which declares param dependencies in .apply and in operations, e.g.:

@pn.depends(field.param.value, agg_fn.param.value)
def aggregator(field, agg_fn):
    field = None if field == "counts" else field
    return agg_fn(field)

datashade(..., agg=aggregator)
# or
obj.apply(datashade, agg=aggregator)

This complements the ability to use functions with dependencies as DynamicMap callbacks.

Cc: @jbednar

[jbednar]

Excellent! I can't test it because my new laptop doesn't have Xcode yet and I'm having Apple ID problems, but it certainly looks like it would work...

[jbednar]

Ok, I've tested it and it works perfectly! You can see the result at https://anaconda.org/jbednar/dashboard_barewidgets , comparing it to the Param-based version at http://datashader.org/dashboard.html . The main remaining awkwardness in that notebook is the constant .param.value invocations, which would be addressed by #3976 and changes already in Panel master that will be in 0.7.0.