Brightway2

* FEATURE: Add class and mixin for Monte Carlo using direct solvers
* CHANGE: Move tests to root directory and add Monte Carlo tests
* CHANGE: Consistent use of `__next__` and `next()` so that all Monte Carlo iterator classes are Py2/3 compatible and programmed the same way. `ParameterVectorLCA.next()` will no longer work on Python 3; instead, call `next(ParameterVectorLCA)`. When providing a new vector, call the class itself (after it is instantiated): `pv = ParameterVectorLCA(args); pv(new_vector)`.

* CHANGE:Updates for bw2data 2.2
* BUGFIX: Correctly handle regionalized CFs in site-generic calculations
* FEATURE: Add contribution methods to LCA classes

BUGFIX: Correctly handle project names in multiprocess calculations

BUGFIX: `switch_*` was seriously broken due to new handling of processed arrays filepaths

* Feature: Py3 compatibility
* FEATURE: Independent LCAs which don't rely on bw2data and the brightway2 ecosystem
* Feature: Add `DenseLCA`
* FEATURE: Added `switch_*` and `to_dataframe` methods to LCA class
* FEATURE: Allow graph traversal to skip links in static databases
* BUGFIX: Terminate multiprocessing pools after calculations
* CHANGE: Load data automatically in Monte Carlo
* CHANGE: Automatically clean dirty databases before starting calculations

Plus lots of small bugfixes, and compatibility with `projects`.

- BUGFIX: Don't die if `xlsxwriter` not installed.