Add starting locations for transported molecules

[ukemi]

Since we have changed the representation of transport to now use the relation 'has_primary_input', we should be more explicit about the input molecule. Here is an example of a transport reaction from pyruvate metabolism: R-HSA-70268

It would be nice if we could represent this like this:

It would be even better to get rid of the start and end location assertions and just infer that by the locations of inputs and outputs, but that is a more general modeling discussion.

[ukemi]

This is related to #200