ImageStack support and aggregator fixes

[maximlt]

Fixes #1315

With these changes:

• rasterize=True, by='cat' is equivalent to rasterize=True, aggregator=ds.count_cat('cat')
• the value of eltype is correctly set to ImageStack when an ImageStack should be returned, so the options are correctly applied to the output of the operation

I've moved the logic that infers whether a datashaded plot is categorical or not and the aggregator to a method that is used in two places.

import datashader as ds
import numpy as np
import pandas as pd
import hvplot.pandas

num=10000

dists = {cat: pd.DataFrame(dict([('x',np.random.normal(x,s,num)), 
                                 ('y',np.random.normal(y,s,num)), 
                                 ('val',val), 
                                 ('cat',cat)]))      
         for x,  y,  s,  val, cat in 
         [(  2,  2, 0.03, 10, "d1"), 
          (  2, -2, 0.10, 20, "d2"), 
          ( -2, -2, 0.50, 30, "d3"), 
          ( -2,  2, 1.00, 40, "d4"), 
          (  0,  0, 3.00, 50, "d5")] }

df = pd.concat(dists,ignore_index=True)
df["cat"]=df["cat"].astype("category")

p = df.hvplot.scatter('x', 'y', rasterize=True, aggregator=ds.count_cat('cat'), width=800)
p