Make units consistent

[mortenengen]

For the moment, the units in e.g. sections, constitutive laws and code equations, are not consistent. This means that the user could input e.g. MPa for stresses, and kN for forces, without any guarantee that an infered value will have meaningful unit. We should explore options for handling this.

There are several options, including:

1. Not at all handling units, and letting it be up to the user to provide input in consistent units, and make sure that whenever unit dependent methods are implemented, the required unit is specified in the docstring.
2. We could set a global set of units that the user can adjust, expect that the user inputs values that are consistent with this set of units, and whenever a conversion is needed, this is performed automatically based on the globally set units.
3. We could set a global set of units like above, and all default values of input with units are handled automatically, such that in all calculations we are sure that the units are consistent, and that the result will have a consistent unit.

At least options 2 and 3 could required that we adapt some of the implemented design code equations to be compatible.

[connorferster]

Maybe use pint or another well-established unit library instead of spending the time rolling your own?

This is just one of the problems/decisions-that-need-to-be-made when creating a units library (ask me how I know!). I would recommend working on making structuralcodes compatible with pint (if it is not already) and then users can handle their own units.

[mortenengen]

Hey @connorferster. Thanks for reaching out, I really appreciate your comment on this issue. If you don't mind, I will continue the discussion on #205 where we have started a draft implementation.