mob: hexagonal position allocator

This merge request has two features:

1. HexagonalPositionAllocator locates Nodes in a hexagonal grid in the X-Y plane, at a fixed Z. All Nodes are equidistant from their neighbors. Hexagonal grids are commonly used in abstract models for wireless (cell-phone) systems, such as PCS, 4G and 5G macro cells.
2. FilteredPositionAllocator, which applies a PositionFilter to an underlying PositionAllocator. PositionFilters can be created from other PositionAllocators, enabling some composition of allocators. In the main-hex-allocator.cc example this is used to sample UE locations from UniformDiscPositionAllocator, then filter them to the exact footprint of the HexagonalPositionAllocator.