Lca-algebraic

* Add multi-dataset support, replacing **SET_USER_DB** by **setBackground()**, **setForeground()**
* Added **freezeParams()** to freeze parametric dataset for specific scenario to be used by non parametric tools or other foreground datasets.
* Added dataset parameters : parameters scoped to a specific dataset, with scope / conflict resolution of parameters with same names.
* Added extended import / export of datasets with their parameters : **export_db()**, **import_db()**
* Added non regression tests

* Added explicit error when cycle is detected in activities.
This usually means a Bg activity has been imported into User DB :
lca_algebraic does not support cycles / recursion in User Db,
since those are developped as litteral formulas.
* Added support for several background databases.
User Db is guessed or can be specified using **SET_USER_DB**()
* Support coproducts in graphtraversal.
See [PR 9] by ntropy-esa
* Add support for loading / saving parameters into Brightway DB, with compatibility with Brightway2 and Activity Browser.
See functions **loadParams**() and **persistParams()**. See also issue 8.
* Added new function **exploreImpacts()**, like **printAct()** but showing impact of each exchange.

* Internal change for generated webapp
* Add several display options to graphs()
* Fix overlapping graph bug

* Fixed bug in param **extract_activities** of **multiLCAAlgebric**

* Added a step of simplification of models in **sobol_simplify_model** by removing minor terms of sums
* Added support for alternate names for impact methods with the function **set_custom_impact_labels**
* Added parameter **extract_activities** to **multiLCAAlgebric** in order to compute contribution of a subset of activities to the global impact

Added **BETA** distribution type. See [online doc](https://oie-mines-paristech.github.io/lca_algebraic/doc/params.html#lca_algebraic.params.DistributionType)