Enable non-constant, per-row tracker tilt angles

[eyoung55]

We would like to be able to test the effect of varying the tracker tilt angle throughout the array, e.g., tilting the front row to -50 deg, tilting the second row to -10 deg, and so on.

Changes:

• Enable array inputs in the schema file for tracker_angle
• Update mesh generation code for 2D panels to check for array inputs
• Update mesh generation code for 3D panels to check for array inputs

Closes: #80