Refactor transformers' tests

[sfalquier]

Transformer's unit test must only test one transformer. All unit tests can share input dataframes when it's relevant, but it's not mandatory.

Depends on #115 to remove input datasets with id/ts/lat/lon when not necessary for the Transformer.

Split this task into several PRs relying on packages created in #84

• [ ] misc
• [ ] tools
• [ ] features
• [ ] timeseries
• [ ] ml

[laurent-thiebaud-gisaia]

Merged in integration branch, to be reopened if necessary after review