Add reverse of secretion syntax sugar

[cthoyt]

Since secretion(X) auto-expands to translocation(X, fromLoc(GO:intracellular), toLoc(GO:"extracellular space")), there should be another automatic translocation() called absorption(X) that is the opposite of secretion and expands to translocation(X, fromLoc(GO:"extracellular space"), toLoc(GO:intracellular))

Add another static tag here:

https://github.com/pybel/pybel/blob/cce8dafd4704a2de2aa95a7073faaa39bf9fd5e8/src/pybel/parser/parse_bel.py#L42-L44

Add a corresponding instance tag here:

https://github.com/pybel/pybel/blob/cce8dafd4704a2de2aa95a7073faaa39bf9fd5e8/src/pybel/parser/parse_bel.py#L254-L258

Register it with the parser here:

https://github.com/pybel/pybel/blob/cce8dafd4704a2de2aa95a7073faaa39bf9fd5e8/src/pybel/parser/parse_bel.py#L293-L299

Add another conditional here:

https://github.com/pybel/pybel/blob/cce8dafd4704a2de2aa95a7073faaa39bf9fd5e8/src/pybel/tokens.py#L430-L440

[cthoyt]

See: https://github.com/belbio/bep/issues/7