Nanoflann: remove internal find, check and build; use find_package with namespace

[pierrepebay]

What I did to "build" nanoflann:

# from MUQ root:
mkdir deps
mkdir deps/nanoflann
cd deps/nanoflann
mkdir build install
# we can change this to .tar if needed
wget https://github.com/jlblancoc/nanoflann/archive/refs/tags/v1.5.5.zip
unzip v1.5.5.zip
cmake -S nanoflann-1.5.5/ -B build/
    -DCMAKE_INSTALL_PREFIX=~/Develop/MIT-MUQ/deps/nanoflann/install \
    -DNANOFLANN_BUILD_EXAMPLES=OFF \
    -DNANOFLANN_BUILD_TESTS=OFF
cd build
make install

How I used this build with MUQ:

cmake -DCMAKE_C_COMPILER=gcc \
      -DCMAKE_CXX_COMPILER=g++ \
      -S /home/plpebay/Develop/MIT-MUQ/ \
      -B /home/plpebay/Develop/MIT-MUQ/build/ \
      -DMUQ_USE_GTEST=ON \
      -DMUQ_GTEST_DIR=/home/plpebay/Develop/MIT-MUQ/deps/googletest/install \
      -DCMAKE_INSTALL_PREFIX=/home/plpebay/Develop/MIT-MUQ/install \
      -DHDF5_DIR=/home/plpebay/Develop/MIT-MUQ/deps/hdf5/install/share/cmake \
      -DNLopt_DIR=/home/plpebay/Develop/MIT-MUQ/deps/nlopt/install/lib/cmake/nlopt \
      -DBoost_DIR=/home/plpebay/Develop/MIT-MUQ/deps/boost/install/lib/cmake/Boost-1.85.0 \
      -DSUNDIALS_DIR=/home/plpebay/Develop/MIT-MUQ/deps/sundials/install/lib/cmake/sundials \
      -DEigen3_DIR=/home/plpebay/Develop/MIT-MUQ/deps/eigen/install/share/eigen3/cmake \
      -Dnanoflann_DIR=/home/plpebay/Develop/MIT-MUQ/deps/nanoflann/install/share/cmake/nanoflann/
make -j24 && make install

[pierrepebay]

A couple notes:

• turning off examples when "building" nanoflann is important as some of them require Eigen
• I commented out the preprocessor test in MUQ/Modeling/Flann/FlannCache.h as I don't set the MUQ_NANOFLAN_PARAMS_COMPILES variable anymore, since the test in https://github.com/NexGenAnalytics/MIT-MUQ/blob/d99f6124bf142922d7973cc60c25d7a518084d12/cmake/checks/CheckNANOFLANN.cmake#L50-L65 has been removed; this isn't an issue with newer versions of nanoflann, but will cause compiling failures if an older version is given.