Location: equals(…) is not symmetric

[thtomate]

While developing a test for my last pull request, I encountered a problem with the comparison of Location objects.
It is not symmetric, i. e., the order of the objects of interest is relevant. This happens when only one of the objects has null as id or coord .

Example:
Define test stations:

Location station_with_name_and_id = new Location(LocationType.STATION, "1576", "Lummerland", "Muehle");
Location station_with_name = new Location(LocationType.STATION, null, "Lummerland", "Muehle");

Now:

station_with_name.equals(station_with_name_and_id)==true
station_with_name_and_id.equals(station_with_name)==false

This is unintuitive to use and mathematically unpleasing.

I guess the problem is at that line (and later for coord resp.):

public-transport-enabler/src/de/schildbach/pte/dto/Location.java

Line 164 in 5f0f872

if (this.id != null)

There are pros and cons on which logical operator to use here when fixing.