Lca-algebraic

Fixed [issue 4](https://github.com/oie-mines-paristech/lca_algebraic/issues/4) : Added back handling of default params

* Removed cache for **find{Bio/Tech}Act**, in order to lower memory usage.
Instead, we now use core Brightway capabilities for searching activities.
* Added function **sobol_simplify_model** for generating simplified models,
and **compare_simplified** for comparing their distributions with the initial model.

Cleanup dependencies

* Added parameters 'var_params' for all stats tools, for providing list of parameters to vary.
Other parameters will be set at their default value.
* Fixed typo in the function name **incer_stochastic_dahboard** => **incer_stochastic_dashboard**
* Fixed bug of int overload : convert everything to float

* Rename **modelToExpr()** to **simplifiedModel()**
* Improve performance of computation. Added multithread computation for LCA and Sobol.
* Added method **sobol_simplify_model** that computes list of most meaningful parameters,
based on Sobol analysis, and then generates simplified models based on it.

* Add function `modelToExpr(model, impacts)`, returning expression with background activities replaced
by the value of each impact.