feat(const): added constants class

constants class can parse and get physical constants from datafile including value, unit, uncertainity, and reference

src/const/meson.build

@@ -0,0 +1,18 @@
+# Define the library
+const_sources = files(
+  'private/const.cpp',
+)
+
+const_headers = files(
+    'public/const.h'
+)
+
+# Define the libconst library so it can be linked against by other parts of the build system
+libconst = library('const',
+    const_sources, 
+    include_directories: include_directories('public'),
+    cpp_args: ['-fvisibility=default'],
+    install : true)
+
+# Make headers accessible
+install_headers(const_headers, subdir : '4DSSE/const')