constexpr construction for Vec classes
Vec class objects can now be constructed during compile-time as constant expressions by calling Vec( T, ... ) constructors or through brace-initialization. Constant expression using fill constructor and nested vectors of sizes greater than 4 are not supported yet. Changes made to WrappedOperators.h for resolving overload ambiguities in Vec construction and typecasting. Appropriate test cases were added to UnitTestTypes.cxx. Addresses issue #199.