SVD Call Fail / Initial State Checks

[piperfw]

If you pass an initial state of all zeros to Tempo.compute the code fails with an IndexError in node_array.py in an SVD call, presumably because the matrix is rank 0.

I think we should raise an exception early and loudly to the user if initial state is zero rather than letting it propagate and returning a long stack-trace.

The main questions would be where to implement a check and how often e.g. in node_array.py every timestep or closer to the user interface for the initial state only.

I also noticed you can pass an ndarray of boolean values for the state. I suppose this is intentional?

MWE attached.

import numpy as np
import oqupy
# Raises IndexError
initial_state = np.zeros((2,2))
# Fine (but non-physical)
#initial_state = np.array([[0.1,0],[0,0]])
# Fine?
#initial_state = np.array([[True,False],[False,True]])

system = oqupy.System(np.eye(2))
correlations = oqupy.PowerLawSD(alpha=1, zeta=1, cutoff=1)
bath = oqupy.Bath(np.eye(2), correlations)
tempo_params = oqupy.TempoParameters(dt=0.1, epsrel=10**(-7))

tempo_sys = oqupy.Tempo(system,
                        bath,
                        tempo_params,
                        initial_state,
                        start_time=0.0)
tempo_sys.compute(end_time=1.0)