Major API changes

1. No match_and_add function now!!
2. Instead, get_closures, get_best_closure and get_top_k_closures available, performing same functions as their name suggests.

The following new features are added:

1. A Ground Alignment module which aligns each local map's physical ground-plane to its xy-plane.
2. A feature pruning step based on the self-similarity of ORB feature descriptors.

The other minor changes are:

1. Modify DensityMap generation based on analysis from heaptrack. Now it allocates less useless memory.
2. Make magic numbers in code as static constexpr
3. make container sizes' types as std::size_t for windows MSVC to not complain
4. Drop pipeline for groundtruth closure computation (This will be made available later as a separate repo, and could be still used from earlier tags/versions of this repo)