Minimize dependencies

[bkuczenski]

The lca_disclosures package should not depend on any specific implementation (like bw2, lcopt, or lca-tools). Ideally, anything a specific subclass needs should be passed as an argument.

Implications:

• importers don't (necessarily) belong in this class, unless they can meet the above requirement
• tests that require implementation frameworks are not really unit tests, but integration tests, and maybe don't belong?

Dependency on pandas and scipy are ok because they concern core class functionality.