Flip sign on waste exchanges in imported database

[PascalLesage]

Rationale: The convention for waste exchanges in some databases,
notably the ecoivent database, is the following:
- waste exchanges produced by an activity are stored as negative
  inputs
- waste exchanges flowing into a waste treatment activity have a
  negative
In SimaPro, produced waste are stored as positive inputs, and waste
treatment datasets have positive outputs.
If left as are, the supply of waste treatment services from the
linked-to database "other" to imported activities would have the
wrong sign.
This function flips the sign on all inputs from database "other" if
the production amount for that exchange in the activity for which it
is the reference flow is negative.

Note: the strategy needs to be run *after* matching with ecoinvent.
Strategy should be run as follows:
sp_imported.apply_strategy(functools.partial(flip_sign_on_waste, other="name_of_other"))

Closing misplaced issue brightway-lca/brightway2-data#66

[PascalLesage]

Tests failing because of #78.
Tests pass locally.