Critical 1-axis tracking update plus unit testing

A sign error in calculating the tracker angle resulted in a 1-hour offset between the calculated tracker angle and actual tracker angle. This resulted in over-estimates of bifacial gain. v0.2.2 fixes this problem. Unit tests are also introduced. These can be run from the command line by running 'pytest' in the repository root folder.

Model intercomparisons look favorable with PVSyst, SAM (beta) and bifacialvf for fixed-tilt conditions.

Single-axis tracking also checks out within 10%-15% (relative) for rear irradiance.