build(libconstants): changed over to use external libconstants

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

src/eos/private/helm.cpp

@@ -41,10 +41,6 @@
 #include "config.h"
 #include "quill/LogMacros.h"
 
-namespace serif::constant {
-  class Constants;
-}
-
 using namespace std;
 
 // interpolating polynomila function definitions
@@ -246,7 +242,7 @@ namespace serif::eos::helmholtz {
     serif::probe::LogManager& logManager = serif::probe::LogManager::getInstance();
     quill::Logger* logger = logManager.getLogger(logFile);
 
-    serif::constant::Constants& constants = serif::constant::Constants::getInstance();
+    fourdst::constant::Constants& constants = fourdst::constant::Constants::getInstance();
     const double pi = std::numbers::pi;
     const double amu = constants.get("u").value;
     const double h = constants.get("h").value;