Added linear operator, mass and identity, and ConjGrad operators, and implementations

Added a linear operator base class for LHS and Preconditioner operations with the conj grad solver, identity operator for testing purposes, mass matrix operator without optimisation

This MR has been partially split into:

• Identity matrix implementation (see !36 (merged))
• Mass matrix implementation (see !35 (merged))

Operators/AssmbScatr/AssmbScatr.cpp

+#include "AssmbScatr.hpp"
+
+namespace Nektar::Operators::detail
+{
+
+// Register implementation with Operator Factory
+template <>
+std::string OperatorAssmbScatrImpl<double>::className =
+    GetOperatorFactory<double>().RegisterCreatorFunction(
+        "AssmbScatr", OperatorAssmbScatrImpl<double>::instantiate, "");
+
+} // namespace Nektar::Operators::detail
\ No newline at end of file