Ghosh implementation for downstream scope 3

[Beckebanze]

See issue #135 for more details.

With this PR the following is added/implemented:

• equivalent of A for Ghosh (often referred to A* in literature)
• the Ghosh inverse (often referred to G in literature). If the Leontief matrix L is already calculated, then G is calculated based on L because the calculation based on A* is computationally much more expensive.
• downstream scope 3 multiplier, M_{down}, such the sum of the M+M_{down} is the full scope multiplier, with M the existing multiplier in pymrio that covers scope 1,2&3 upstream.
• a short addition to the pymrio background page that introduces the Ghosh model
• tests that test the functionality of the added functions