Update `oceans.build_all_MOR_seedpoints` to work better with topological lines.

[jcannon-gplates]

Currently the generation of seed points at mid-ocean ridges (MOR) for age gridding relies on the MOR having the GPML properties reconstruction method, left plate ID and right plate ID.

This is needed to calculate the stage rotation that moves points off the MOR and onto the left and right plates. As well as for calculating their spreading rates.

However, if an MOR is a topological line it typically does not need these GPML properties (for positioning in GPlates, for example) because the topological line depends on its constituent points for positioning. The points themselves could have these properties, but only if that is what determines their positions. However the properties would then be on the points, not the topological line where they're currently needed for age gridding, and so the age gridding would fail here (see #190).

So, those properties should only really need to be present if they determine the position. We should not need to add those properties just to get the age gridding workflow to work properly.

The solution is to use new pyGPlates 1.0 functionality to query the left and right plate velocities at uniform points along MOR plate boundaries. And this does not rely on feature properties. And it also works in the presence of deforming networks (not that they're used at spreading ridges).

From those left/right velocities, we can move the seed points onto the left and right plates. And we can find the left/right spreading velocities - which will also work correctly with asymmetric spreading (unlike the current approach) since we also have the MOR boundary velocity (ie, left_spreading_velocity = left_plate_velocity - MOR_velocity and right_spreading_velocity = right_plate_velocity - MOR_velocity).