ECLIPSE I/O: Rename Namespace Opm::ecl -> Opm::EclIO

This reduces the risk of conflicting with the 'ecl' namespace from
LibECL, especially when the latter name is used from within the Opm
namespace.

tests/test_ERst.cpp

@@ -33,7 +33,7 @@
 #include <stdio.h>
 #include <tuple>
 
-using namespace Opm::ecl;
+using namespace Opm::EclIO;
 
 template<typename InputIterator1, typename InputIterator2>
 bool