Ddf-utils

- new subcommands for `ddf`: `validate_recipe` and `build_recipe`
- include a recipe schema for validating recipes
- documents for new commands
- minor improvments

- bug fix in several procedures
- minor improvments

- new `insert_keys` option to `groupby` procedure
- new `keep_columns` option to `fliter_row` procedure
- new `split_datapoints_by` and `sub_folder` option for serving
- misc improvements and bug fixes

- new options for `extract_concepts` procedure (40)
- the `key` parameter for ingredients now only accepts string (39)
- Note: this will break recipes worked on pervious version with entities
ingredients which `key`s are lists.
- `translate_column` can ask for user input when ambiguity found (34)
- the ingredient dataframe's dtype will set according to the concepts table (43)
- added debug options to all procedures (46)
- added `trend_bridge` procedure (42)
- code cleanup and formatting
- added lots of documents

- added tests for all procedures
- removed outdated procedures (align, accumulate)
- more api docs, and all docs are available in read the docs now
- new `--ddf_dir` option for `ddf run_recipe` 45
- add options for `serve` procedures and `serving` section. Now you should
provide a list of dictionaries in `serving` section, instead of a list of
ids as pervious version
- minor improvements and bug fixes

- new proecedures: `window` (25)
- updated `groupby` procedure (25)
- updated `translate_column` procedure to include the function in `align` (3)
- minor improvements