Loop closure base

Here is part of the work of Tessa, former intern @PAL.

1. A base class for loop closure processor
2. A derived class for searching loop closure candidate based on frame covariance overlap

added 9 commits

• 1288bb7e...760c5941 - 8 commits from branch master
• f66a218a - Merge branch 'master' into loop_closure_base

Compare with previous version

merged

mentioned in commit 3c1d095a