Propogation/low latency (don't merge yet, just review)

Before merging we need to make more room in memory for the intermediate set of base_obss_t. We'll do that by getting rid of some of the matrix quadratic forms that are constructed in memory.

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We now have a message defined that the base station can send to identify its known position and its already being decoded and used in `solution.c`. This should supersede any other calculation of the base position.

Check out the globals `base_pos_known` and `known_base_ecef`. `base_pos_known` is a `bool_t` that indicates if the ECEF coordinates are valid and usable.

There is an argument for factoring out the calculation / filtering / sanity checking of the base position into its own sub-system as it may be used in multiple places but this can wait for later, perhaps add a TODO now though.