CUDF gpu memory management #17

[mikehinchey]

Allow config OMNISCI_DB_HOST and OMNISCI_DB_PORT for testing.

pyarrow>=3.0.0 and thrift<=0.14.0 in environment_gpu.yml and environment.yml to be consistent with cudf (pyarrow 5.0.0)

Fix test_invalid_sql because error message from DB has changed.

pyproject.toml build reqs so pip install works.

Add a Makefile for other common commands.