Add runtime type checking to Batch class

[khk-globus]

Per user-identified typo-able interaction:

args=("abc")   # a str
args=("abc",)  # a tuple

Python syntax "gotcha". So, help users of this interface with a little judicious type checking.

While here, add a unit test of the Batch class, which we seem to have been missing. Doh!

Type of change

• New feature (non-breaking change that adds functionality)
• Code maintenance/cleanup