Create domain models for Trail, Trail Segment, and Trail Head

[embario]

We need to encapsulate the structure for each of these entities and pull together functionality for querying such data efficiently and with clarity in the rest of the code.

[masnes]

Some of my notes so far looking through the code:

Need a TrailData:
Stores state of trail data
When last fetched
If a fetch is in progress
And the data itself

Need a TrailSegments:
stores segments and data related to them
Stores if segment fetching is in progress

Need a TrailLayers

Need a TrailFilters