Add automatic relation of SO line and logisitc route based on sourced_by field.

Wee look for the PO related to current SO line by the sourced_by fields.

If the PO has a destination location with usage "customer" we apply the dropshipping route to SO line.

If the PO has a destination location with usage "internal" we apply the make to order route to SO line.
to help me understand a failing test, rephrase it

To figure out what is the assertion, I used some harmless gherkin-like
wording.
adapt the test to test the new onchange

We are now green again, and the test looks more readable to me.

On the other hand, we lost the test for the warning in case of
inconsistent PO destination as SOL route.

I am not convinced this check is still necessary. It does no harm still:
we seem to always follow the happy path.
document why the dropshipping dep was added again
Merge pull request #56 from nbessi/routes_in_so_line

Add automatic relation of SO line and logisitc route based on sourced_by...