Fixed typing of arithmetic methods

[RobbieKiwi]

Closes #435.

Changes proposed in this Pull Request

Arithmetic methods now understand that data array is a valid type for addition/multiplication

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[lkstrp]

Thanks @RobbieKiwi, that was annoying. Could you also add the missing lines to the test and resolve the other mypy warnings?

[RobbieKiwi]

Is there a reason why one of these should work and the other should give a type error?
x + x * y + 5
5 + x * y + x

[RobbieKiwi]

@lkstrp I went down a bit of a rabbit hole with the typing in order to make sure the type checker could tell the difference between a LinearExpression and QuadraticExpression which involved breaking the inheritance between the two and making a base class. Could you please have another look?

[FabianHofmann]

impressive work @RobbieKiwi ! @lkstrp or I will go through it soon! we'll make another release today but this will definitely go into the next one!