Driver plugin method -- use entry_points

[ceholden]

Abandon finding time series drivers through AbstractTimeSeries subclassing in favor of setuptools's entry_points.

• should save some load time and is more straightforward for drivers distributed with TSTools.
• avoids class inheritance gore
• setuptools entry_points allows for driver plugins to register under 'TSTools.drivers' so 3rd party ones may be added by installing a Python package

[ceholden]

Closed by 7ec87bd