partial build fix

Still open - I'm missing the tensor background, but a tensor4 with 0 tensors doesn't seem logical. Can you please try to fix this timely, as it's blocking other stuff?

```
[  2%] Building CXX object CMakeFiles/vmmlib_test.dir/tests/tensor4_test.cpp.o
In file included from /Users/eile/Software/Buildyard/src/vmmlib/tests/tensor4_test.cpp:4:
/Users/eile/Software/Buildyard/src/vmmlib/include/vmmlib/tensor4.hpp:457:36: error: comparison of unsigned expression < 0 is always false [-Werror,-Wtautological-compare]
            for( size_t i4 = 0; i4 < I4; ++i4 )
                                ~~ ^ ~~
/Users/eile/Software/Buildyard/src/vmmlib/include/vmmlib/tensor4.hpp:540:13: note: in instantiation of member function 'vmml::tensor4<5, 4, 6, 0, long>::fill' requested here
            fill( static_cast< T >( 0.0 ) );
            ^
/Users/eile/Software/Buildyard/src/vmmlib/include/vmmlib/tensor4.hpp:340:13: note: in instantiation of member function 'vmml::tensor4<5, 4, 6, 0, long>::zero' requested here
            zero();
            ^
/Users/eile/Software/Buildyard/src/vmmlib/tests/tensor4_test.cpp:233:42: note: in instantiation of function template specialization 'vmml::tensor4<5, 4, 6, 0, long>::tensor4<3, 2, 4, 2>' requested here
        tensor4< a+2, b+2, c+2, d-2, T > t4resized(t4);
                                         ^
In file included from /Users/eile/Software/Buildyard/src/vmmlib/tests/tensor4_test.cpp:4:
/Users/eile/Software/Buildyard/src/vmmlib/include/vmmlib/tensor4.hpp:481:57: error: comparison of unsigned expression >= 0 is always true [-Werror,-Wtautological-compare]
            if ( i1 >= I1 || i2 >= I2 || i3 >= I3 || i4 >= I4 )
                                                     ~~ ^  ~~
/Users/eile/Software/Buildyard/src/vmmlib/include/vmmlib/tensor4.hpp:350:29: note: in instantiation of member function 'vmml::tensor4<5, 4, 6, 0, long>::at' requested here
                            at(i1, i2, i3, i4) = source_(i1, i2, i3, i4);
                            ^
/Users/eile/Software/Buildyard/src/vmmlib/tests/tensor4_test.cpp:233:42: note: in instantiation of function template specialization 'vmml::tensor4<5, 4, 6, 0, long>::tensor4<3, 2, 4, 2>' requested here
        tensor4< a+2, b+2, c+2, d-2, T > t4resized(t4);
                                         ^
In file included from /Users/eile/Software/Buildyard/src/vmmlib/tests/tensor4_test.cpp:4:
/Users/eile/Software/Buildyard/src/vmmlib/include/vmmlib/tensor4.hpp:188:35: error: comparison of unsigned expression < 0 is always false [-Werror,-Wtautological-compare]
            for(size_t i4 = 0; i4 < I4; ++i4)
                               ~~ ^ ~~
/Users/eile/Software/Buildyard/src/vmmlib/tests/tensor4_test.cpp:270:13: note: in instantiation of member function 'vmml::operator<<' requested here
            << t4resized
            ^
In file included from /Users/eile/Software/Buildyard/src/vmmlib/tests/tensor4_test.cpp:4:
/Users/eile/Software/Buildyard/src/vmmlib/include/vmmlib/tensor4.hpp:213:29: error: comparison of unsigned expression < 0 is always false [-Werror,-Wtautological-compare]
                if ( i4 + 1 < I4 )
                     ~~~~~~ ^ ~~
/Users/eile/Software/Buildyard/src/vmmlib/include/vmmlib/tensor4.hpp:495:57: error: comparison of unsigned expression >= 0 is always true [-Werror,-Wtautological-compare]
            if ( i1 >= I1 || i2 >= I2 || i3 >= I3 || i4 >= I4 )
                                                     ~~ ^  ~~
/Users/eile/Software/Buildyard/src/vmmlib/include/vmmlib/tensor4.hpp:200:46: note: in instantiation of member function 'vmml::tensor4<5, 4, 6, 0, long>::at' requested here
                                os << int(t4.at( i1, i2, i3, i4 )) ;
                                             ^
/Users/eile/Software/Buildyard/src/vmmlib/tests/tensor4_test.cpp:270:13: note: in instantiation of member function 'vmml::operator<<' requested here
            << t4resized
            ^
5 errors generated.
```