Brightway2

* Merge [119](https://github.com/brightway-lca/brightway2-io/pull/119): Fix variable error in ecospold1 extraction
* Fix [124: Custom output dir for Excel/CSV export](https://github.com/brightway-lca/brightway2-io/issues/124)
* Fix [131: Issue when importing CSV inventories](https://github.com/brightway-lca/brightway2-io/issues/131)
* Normalize field `reference unit` during unit normalization
* JSON-LD LCIA importer: Can't assume some fields are present

* Change label for chemical formulas in ecospold2 import from `formula` to `chemical formula`
* Add mathematical formula field `mathematicalRelation` from ecospold2 imports as `formula`
* Add variables names for exchanges and exchange properties in ecospold2 imports
* Add strategy to lookup chemical synonyms in [ChemIDPlus](https://chem.nlm.nih.gov/chemidplus/)

* Don't export `id` field in Excel/CSV

* Continued work on JSON-LD imports
* JSON-LD LCIA importer
* Shortcut to import US EEIO 1.1 database and LCIA methods

* Partial support for JSON-LD imports

* Compatibility with ecoinvent 3.9 LCI & LCIA