Support 2+ base stations for crossing beam method

[krichardsson]

In the Lighthouse system, there is the option to use the crossing beam method to compute the current position. This was originally implemented for LH1 and currently it only works for 2 base stations, id 1 and 2. In a LH2 it is now possible to use more than 2 base stations, but base stations with an id > 2 will be ignored.

It is probably not very hard to extend the implementation to a crossing beam computation between arbitrary base stations.

For the user, it is not obvious why the CF does not get any position estimate when base station 1 & 2 are not visible.