communication: fix modulo (#1356)

If `m_coordines[x] == 0` and `X == -1`, we might do `-1 % N`. The sign of the result was implementation-defined pre-stdc++11; in GCC 7.3 this is incorrectly still negative, which leads to problems with MPI later in the code.

Backport of #1355

Author: lukasm91

include/gridtools/communication/low_level/proc_grids_3D.hpp

@@ -183,23 +183,23 @@ namespace gridtools {
             int _coords[3];
 
             if (m_cyclic.value(0))
-                _coords[0] = (m_coordinates[0] + I) % m_dimensions[0];
+                _coords[0] = (m_coordinates[0] + I + m_dimensions[0]) % m_dimensions[0];
             else {
                 _coords[0] = m_coordinates[0] + I;
                 if (_coords[0] < 0 || _coords[0] >= m_dimensions[0])
                     return -1;
             }
 
             if (m_cyclic.value(1))
-                _coords[1] = (m_coordinates[1] + J) % m_dimensions[1];
+                _coords[1] = (m_coordinates[1] + J + m_dimensions[1]) % m_dimensions[1];
             else {
                 _coords[1] = m_coordinates[1] + J;
                 if (_coords[1] < 0 || _coords[1] >= m_dimensions[1])
                     return -1;
             }
 
             if (m_cyclic.value(2))
-                _coords[2] = (m_coordinates[2] + K) % m_dimensions[2];
+                _coords[2] = (m_coordinates[2] + K + m_dimensions[2]) % m_dimensions[2];
             else {
                 _coords[2] = m_coordinates[2] + K;
                 if (_coords[2] < 0 || _coords[2] >= m_dimensions[2])