To allow for the generic get_meta, set_meta, delete_meta methods,
rename the scenario specific meta methods.

- update ixmp.jar
- add Platform.get_meta, Platform.set_meta
- add some tests (only two pass so far)

TODO: make all tests pass, maybe add more tests

- remove redundant backend methods from platform
- use unwrap to handle BigDecimals
- fix tests
- replace ixmp.jar (freshly built from appropriate branch)

- look for more specific errors
- add new test for Model+Scenario Meta

- set_meta void return type
- set meta version parameter type

- implement Platform.remove_meta
- deprecate Scenario.delete_meta (Scenario.remove_meta should be used)
- add more tests

- update jar

- add tests

- add release notes
- update docstrings
- remove unused backend method delete_scenario_meta

- update sphinx documentation
- consistently use "meta category" instead of "meta attribute"
- update release notes

To be more consistent with existing Platform methods like
.regions() and .units():
- rename Platform.list_models to Platform.models
- rename Platform.list_scenarios to Platform.scenarios

- Platform.get_meta() does not allow empty arguments
- update ixmp.jar with bugfix of
  https://github.com/iiasa/ixmp_source/pull/329
- update and add tests

describe valid args of getting/setting/deleting meta indicators

- source diff https://github.com/iiasa/ixmp_source/compare/2c3244517d6ffd6de70c896d3198d9d3a43ddfae...07b82efe90a6590b30cc764804f1e9187e9cfb55

'strict' will only retrieve meta indicators from the exact given level
(model, scenario, version combination).

- to distinguish from Scenario/Model objects, rename methods to
  get/retrieve scenarios/models to scenario_name and model_name
- adapt documentation of parameters

- Adapt to renamed method names
- some minor documentation fixes/updates

- update release notes to changed method names
- remove methods from backend test

- correct method name to get_model_names() from model_names()