Use of assert

[davidwilby]

As discussed:

As far as I understand, assert in "production" code isn't advised. This is mainly because when run in optimized mode python -0 <script.py> (or via some other means), the internal __debug__ variable is set to False and stops respecting assert statements. (See official docs: https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement)

assert is essentially:

if __debug__:
    if not expression: raise AssertionError

Now, this is potentially a minor case, but if used in a production setting, it's possible that PolarRoute could be used with python's optimized mode and fall over.

Instead of assert, errors should be handled explicitly with raise (as far as I understand it).