after the rename of "grid" to "cell", the name cell should not be used
for the coordinates of the cell centers. In agreement with the names
x_p/u_p for point positions/displacements, now the "point" is used to
refer to the materialpoints (i.e. cell centers)
Additionally, "_node"/"_point" are now suffixes to
"coordinates"/"displacements".
Finally, "coords" is renamed to "coordinates"
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