build(libconstants): changed over to use external libconstants

this is the same module as before just broken out into its own subproject

tests/meson.build

@@ -4,7 +4,6 @@ gtest_main = dependency('gtest_main', required: true)
 gtest_nomain_dep = dependency('gtest', main: false, required : true)
 
 # Subdirectories for unit and integration tests
-subdir('const')
 subdir('meshIO')
 subdir('probe')
 subdir('config')