Fix pytests and make TS conversion conditional

[zhilingc]

Addressing issues #204 and #195.

Changes made for #204:

• Advanced pandas required version to >=0.24.0 and added fastavro to test-requirements.yaml
• Removed test tests/sdk/utils/test_bq_utils.py::test_query_to_dataframe because it interfaces with BQ and is therefore not a unit test.
• Changed the python sdk to use pandas nullable int types instead of numpy int types
• BQ get table util now does not include granularity in the table name
• Changed importer expected type from file to file.csv, removed format from options

Changes made for #195:

• Added conditional checking if the timestamp column is declared, if not, will not run timestamp conversion
• Added test for that case

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[zhilingc]

/retest

[davidheryanto]

/lgtm