Raise exceptions instead of using assertions

[MocktaiLEngineer]

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https://github.com/octoenergy/xocto/blob/e0446deeb17e333a95d1c27cbf268698efbde3f6/xocto/numbers.py#L21

Assertions can't be relied upon for control flow because they can be disabled, as per the following -

$ python --help
usage: python [option] ... [-c cmd | -m mod | file | -] [arg] ...
...
-O     : remove assert and __debug__-dependent statements; add .opt-1 before
         .pyc extension; also PYTHONOPTIMIZE=x
-OO    : do -O changes and also discard docstrings; add .opt-2 before
         .pyc extension
...

Would you be open to me changing these assertions to other appropriate exceptions as I encounter them?

[samueljsb]

Hi! Sorry for leaving you hanging for so long. You're absolutely right about assertions being no good here. We'd love to see this fixed to raise a better exception.

[thejasmeetsingh]

Hey @samueljsb Just raised a PR with regards to this issue: #137