Support for multiple protein pools

[carrascomj]

Description

geckopy currently supports adding a global protein pool or a protein pool to a handful of specified reactions but does not easily allow for adding multiple protein pools at the same time.

Multiple protein pools are wanted in mainly two assumptions/situations:

1. The maximum capacity for heterologous expression is known, being a subset (or not) of the global protein pool.
2. The comparments in a (eukaryote) model have associated maximum capacities, resulting in different protein pools, being subsets (or not) of a global protein pool.

Proposition

• Graph data structure for the protein pools. One protein pool can take up to a percentage of its parent pool. There may be more than one disconnected graph (for when a global protein pool is not required).
• The user constrains the pool (constrain_pool()) providing a set of proteins (like it is now, by the default all) and a parent pool (by default None, meaning the pool is a root). If a parent is provided, the pool will be taking p_total from the parent pool.