Add elevation nod sequence

[BrianJKoopman]

This PR adds the elevation nod sequence, sorunlib.seq.el_nod(). It will perform the elevation nod as described in #207.

Example usage:

import sorunlib as run
run.initialize()
run.acu.move_to(az=180, el=60)
run.seq.el_nod(el1=50, el2=70)

Resolves #207.