The existing code compared the output of data->size(), which is a
std::vector<double>::size_type (typically std::size_t) and therefore an
unsigned integer type to the number 'grid.number_of_cells' which is a
(signed) 'int'. This leads to an annoying warning when increasing the
warning level in GCC.
While here, also insert code to verify that the 'stride' is a positive
number lest the subsequent assignment loop reference (*data) elements
out of bounds.
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