Added glm as submodule from official repo

Submodule points at release tag 0.9.8.3

.gitmodules

 [submodule "3rdparty/pybind11"]
 	path = 3rdparty/pybind11
 	url = https://github.com/pybind/pybind11.git
+[submodule "3rdparty/glm"]
+	path = 3rdparty/glm
+	url = https://github.com/g-truc/glm.git

glm @ 6a1e2ec5

+Subproject commit 6a1e2ec5d5e79e6d869c947cbdbcbb297bdf9d32

CMakeLists.txt

@@ -95,7 +95,7 @@ message(STATUS "Eigen3 include dir found at ${EIGEN3_INCLUDE_DIR}")
 message(STATUS "Eigen3 version: ${EIGEN3_VERSION}")
 
 set(CEREAL_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/3rdparty/cereal-1.1.1/include")
-set(glm_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/3rdparty/glm-git-modified")
+set(glm_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/3rdparty/glm")
 
 # Header files:
 set(HEADERS