add package dependencies to nytaxi example (#3793)
add package dependencies to nytaxi example (#3793)

* add package dependencies to nytaxi example

when running the app, python will explain about missing imports
(dask and xyarray), but the other missing dependencies (fastparquet
and python-snappy) are more difficult to find, since the `bokeh serve`
command will simply show a white page with no exception on the
command line.

The dependencies listed in the PR are the ones needed in addition if
holoviews was installed via the recommended way:

 conda install -c pyviz holoviews bokeh

* replace dask / xyarray by datashader dependency

ioam/holoviews,ioam/holoviews,ioam/holoviews