cpp mode methods

[henryiii]

In cpp mode, I've been using methods instead of properties to exactly mimic C++. This will (still) cause a compromise somewhere: you can't set metadata without coming up with a new method or making it a property. Here are the options:

1. All argumentless methods are properties, like in Python mode. Note we also have .at(), which has an identical problem, but can't be made into a property.
2. ax.metadata is a property, all others are methods.
3. ax.metadata() is a method, and ax.metadata_set(value) is available (name TBD), similar to at/at_set (name TBD). No methods are turned into properties.

I rather like number 3 best, I think, but no clear winner for me. @HDembinski ? Also, if there are any other things in C++ that support x.y() =, those would fall in this category with metadata.

[HDembinski]

Please go for number 2. We don't know yet, maybe some argument-less method will get an argument in the future, and then that would be a breaking change if the method is a property in Python. For .metadata it is guaranteed that there won't be an additional argument.

[HDembinski]

Regarding .at(), I am very ok if assignment is simply not supported and setting only works via __getitem__

[henryiii]

Sounds good. Metadata is slightly special in Python, anyway, since it can store Python objects.

(Technically, there has to be an underlying method for the __getitem__ to call, so there will always be one hidden away)

[HDembinski]

.metadata is also special in that it is the only thing that you can set on an axis after it was constructed