Function to convert string or list of strings to list of strings

[rstoneback]

Is your feature request related to a problem? Please describe.
We have had a few discussions, and implementations, about dealing with string inputs.

Describe the solution you'd like
A function in pysat.utils that converts inputs to a list of strings. There are at least two so far, one in _instrument.py, the other in _files.py that are already very close.

        starts = np.asarray(start)
        if starts.shape == ():
            starts = [starts.tolist()]
        elif starts.shape[0] > 1:
            starts = starts.squeeze().tolist()
        elif starts.shape[0] == 1:
            starts = starts.tolist()

and

            # User provided some inputs
            starts = np.asarray([start])
            stops = np.asarray([stop])

            # Ensure consistency if list-like already
            if len(starts.shape) > 1:
                starts = starts[0]
            if len(stops.shape) > 1:
                stops = stops[0]

Describe alternatives you've considered
Leave as is. Makes writing functions harder and leaves a potential area to discuss.

Additional context
Distinguishing strings from arrays would be great.

[rstoneback]

Call it listify (suggestion from @aburrell)