4D coordinates

[landswellsong]

We need LineStrings to have 4 coordinates in order to painlessly determine the time when the satellite path intersects the selection geometry. 4th coordinate is meant to be used for linear referencing, but we could in principle reuse it for time values i.e. seconds passed since the start of the session.

The Django's GEOS implementation doesn't support anything with more than 3 coordinates even though the underlying GEOS library does.

As a temporary countermeasure, I'm using the Z coordinate for storing time values. Altitude filters are thus not implemented and Cesium draws wrong altitude.

I see the following solutions so far:

1. Patch upstream django to support 4D LineStrings.
2. Use Z coordinate for time, store altitudes separately.