fixed compile error when eigen3 not installed
Cleaning unit tests imports: Removed unneeded #include, i.e. <init.h>
Cleaning unit tests imports: Don't include Labels, when only Math was wanted.
Cleaning unit tests imports: Don't just include what's working.  Include what's needed.
Merge pull request #2008 from sunil1337/compile_error

fixed compile error when eigen3 not installed
Merge pull request #2011 from tklein23/clean_unittest_imports

Clean unittest imports (removed obsolete imports as a first step)
Added const qualifier to CDistance::get_{lhs, rhs}
Very Rough; Added lib/nanoflann with adaptor
Incomplete; restored Distance.h, changed nanoflann.hpp
Added KDTree calls within KNN.cpp
Merge branch 'develop' of https://github.com/dhruv13J/shogun into develop

Conflicts:
	src/shogun/lib/nanoflann/nanoflann.hpp
	src/shogun/lib/nanoflann/nanoflann_shogun.h