Add `.report.key`

[khaeru]

In message_data.model.transport, we have a submodule .key that does nothing more than define a list of genno.Key. This makes it easier to prepare reporting calculations.

By analogy, add message_ix.report.key with full-dimensionality keys for every set, parameter, and variable available in a message_ix.Scenario via message_ix.Reporter. So for instance instead of this:

from message_ix import Reporter, Scenario

scen, mp = Scenario.from_url("…")
rep = Reporter.from_scenario(scen)

####

from genno import Key

# Operate on full-dimensionality `input` summed on `time`, `time_origin`
k = Key("input", "nl t yv ya m no c l h ho".split())
rep.add("check", "my_operator", k / ("h", "ho"), ...)

…a user could do this:

from message_ix.report.key import input

rep.add("check", "my_operator", input / ("h", "ho"), ...)

This essentially saves having to look up, retype, or use rep.full_key() to get the full dimensionality.