Feature: panama_prices backwards

[JoaoAparicio]

Currently core.instrument.Instrument has a method panama_prices() which computes the panama_prices going forward.

Feature: create a new function, or a boolean argument passed into panama_prices(), which computes the panama_prices backwards (i.e. starting from the most recent date and going back).

[chrism2671]

Not sure there's much point to this, apart from perhaps making it 'look good'?

A quick solution would be to transpose the entire panama series by today's current price diff()