Add simplest possible lattice string for `Tao` ease-of-instantiation and testing

[ken-lauer]

Now, it would be helpful to have a simplest possible lattice str available in the package like:

simple_lattice_str = """
parameter[geometry] = open
beginning[beta_a] = 1
beginning[beta_b] = 1
beginning[e_tot] = 10e6
d: drift, L = 1
lat: line = (d)
use, lat

This could be used for testing.

Originally posted by @ChristopherMayes in #137 (comment)