ENH: Constants

[ksunden]

closes #734

To do:

• interface to set constants that exist (analogous to transform for axes, but may want to make adding and subtracting individual items the primary way)
• Add constants in chop/split where appropriate
• Update constants in remove/rename variable
• Add constants in relevant from methods (perhaps to be broken up into separate PRs for each)
• Floating point representation (currently default number of decimals)
• Tests
• Documentation

No longer planning on adding constants in from_methods, those will come when they are useful/have good heuristics for what to add.

[ksunden]

Any opinions on how removing variable contained within constants should behave?

Options are:

• remove the constant
• raise an error, preventing removal (as happens with axes)

[ddkohler]

If I understand constants correctly, they are not as fundamental to the data object as axes are, so I don't think an error is necessary. Nonetheless, I can imagine users removing these variables inadvertently. Perhaps allow users to remove, but raise a warning?

[untzag]

I agree with @p770193 - a visible warning seems appropriate

[ksunden]

Any opinions on how Join should handle constants? (Need to merge from master to make those changes if any are required)

While I haven't tested, I think join would currently inherit constants from the first data's attrs.
options include:

• Drop all constants
• Drop constants if the variables involved are in the axes that are being joined over
• Include all the constants and maybe have some nonsensical ones
• leave it as is, and see if anybody has issues/desired behavior from experience using