…rty for `TrialData` and `ExperimentData` (#648)

Useful for grouping trials by the config they used. In use by upcoming
#633 for generating graphs with variance error bars for repeated
configs.

Also refactors a number of other things:
- Standardize on `experiment_id` instead of `exp_id`
  (but not the db schema for now)
- Standardize on `tunable_config_id` instead of `config_id` in the API
since we also call it `tunable_config` for the object fetching property
to distinguish from the `config` dict used internally.
  (but not the db schema for now)
- Rework the idea of `TunableConfigTrialGroup` as an object inaddition
to an ID (further methods can be added later to move back and forth
between types when doing interactive analysis).
- Rework the idea of a `TunableConfig` as an object for fetching tunable
value assignments (similar justification - easier grouping in the future
by fetching trial across experiments based on config - eventually could
be used to house the experiment merge logic).
- Rename `results` APIs to `results_df` (similar for others that return
`pandas.DataFrame`) to match the `results_dict` that return `dict`
- Refactor test fixtures to match other styles and for future use (moved
to #644).
- Expand tests

NOTE:
- We cut a new version with this commit since there are potentially
breaking API changes (e.g., `results` -> to `results_df` and `exp_id` ->
`experiment_id`).

Currently builds off of #644 and splits work out of #633

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Co-authored-by: Sergiy Matusevych <[email protected]>