Load from and save text/binary function (for Orbslam2, orbslam3), cmake export target

[siposcsaba89]

Hi,

I have just added some load/save functions used in orbslam repositories, and also wrote a loadFromBinary which very fast. Also updated the CMakeLists.txt to support export/import target. If you have the time, you may take a look at it.

Thanks for the library.