Allow properties to specify monotonicity constraints (e.g. height increasing)

[mcgibbon]

A common property of components is requiring that the vertical dimension has a certain ordering, either starting at the bottom or the top of the domain. We can frame this in a general way by saying the component requires that a certain input (e.g. height or air_pressure) has a certain ordering (e.g. "increasing" or "decreasing") along a certain dimension (e.g. "mid_levels").

I propose this be indicated in the input properties dictionary following this example:

input_properties = {
    'air_pressure': {
        'dims': ['mid_levels', '*'],
        'units': 'Pa',
        'constraints': {'mid_levels': 'decreasing'},
    }
}

I'm open to suggestions on a better word than "constraints". Sympl would check to see that all constraints present for each dimension are either already satisfied (trivially if there aren't any), or would be satisfied if the dimension was flipped (in which case it would do so) or cannot be satisfied (in which case it would error).

[JoyMonteiro]

I think this would be a good property to add, and could also be used to enforce nonzero for example. constraints sounds ok, but requirements might be clearer.

[mcgibbon]

I'll push this back to the next version after 0.4.0. I think requirements like nonzero should be accommodated by allowing functions to be provided which take in the quantity as input and return True if it's satisfied and False if not. For example nonnegative would be lambda x: x>=0. We would do the np.all call when necessary within the Sympl code.

[JoyMonteiro]

you mean allow constraints/requirements to accept a dictionary whose values can be functions?

[mcgibbon]

I mean where there would be the string 'nonnegative' you could instead give a function. I'm not quite sure the best place to put such a requirement.

[JoyMonteiro]

yes, that's what I meant. sounds good. what do you mean by best place to put such a requirement? you mean the correct place to put the function? I think replacing the string with a function sounds just fine to me.

[mcgibbon]

I mean I don't know the best way to specify all of this in a property dictionary. I'm thinking about it.