Add user-interface for Runtime UDF registration

[pearu]

The latest OmnisciDB release contains support for Runtime UDF registration. Python users can use the rbc package for registering Python functions as Runtime UDFs in OmnisciDB server.

This issue proposes adding Runtime UDF registration user-interface to pymapd. The proposal would consist of the following tasks:

• Add __call__ method to Connection class so that the Connection instance can be used as a decorator on Python functions.
• Add UDF registration call (provided by rbc) to all query methods like select_ipc_gpu, etc.
• Add rbc package optional dependency to pymapd. While rbc is a pure Python package, it depends on the following packages: numba, llvmlite>=0.29, tblib, thriftpy2, and six. The rbc optional dependency means that the rbc would be imported within Connection.__call__ method only, that is, pymapd would be functional even when rbc is not installed.

As a result, here is how a typical workflow of using Runtime UDFs would look like:

from pymapd import connect
con = connect(user="admin", password="HyperInteractive", host="localhost", dbname="omnisci")

# Define a Runtime UDF that computes the sum of its arguments:
@con('int32(int32, int32)')
def totaldelay(depdelay, arrdelay):
    return depdelay + arrdelay

# Running the query would preceed a UDF registration call (when UDFs have been defined).
df = con.select_ipc_gpu("SELECT depdelay, arrdelay, totaldelay(depdelay, arrdelay) FROM flights_2008_10k LIMIT 100")

Are there any suggestions or concerns about the proposed UI for Runtime UDFs?

[randyzwitch]

My biggest concern about adding these as dependencies is that we're trying to maintain pip/PyPI installation. Are any of these packages conda only?

[pearu]

All these packages are available via pip. The rbc itself is available via pip under the name rbc-project.

[randyzwitch]

Some of them are dependent on Cython? That's the biggest issue that caused us to re-write the IPC code, people were having a hard time installing pymapd via pip (especially on Windows)

[pearu]

thriftpy2 has optional Cython dependency that is disabled for non-linux platforms.
I guess the best answer comes from trying it out on Windows.

[pearu]

I have updated rbc to work on Windows. Also installing rbc via pip seems to work fine (tested on Windows).

[randyzwitch]

Please go ahead and start working on this @pearu, and before we merge we can do a final pip/conda & linux/osx/windows check

[randyzwitch]

This was implemented in #272