Change "facing west" criterion to allow small uncertainty in `sazm`

[kandersolar]

Closes #61

Using the same reproducer as #61 with this branch, the swapping is gone:

Disclaimer: I'm not very familiar at all with bifacialvf and have not evaluated the suitability of this fix beyond making the above plot.

[kandersolar]

Test failure seems unrelated.

[mikofski]

See my comment - I think checking the sign of sine is a more robust solution.