nanoflann: fix compile error

It is unnecessary to specify the template types of std::make_pair,
as types are automatically deduced by the arguments.
Using C++11, this lead to a compile error.
make typedefs public; fix missing const

A private typedef lead to a very hard to debug compile error
in BOOST_FOREACH elsewhere.
Merge pull request #6 from thorbenk/compile_fixes

Compile fixes