Fix crash when reading gmsh22 files with multiple cells

[stefsmeets]

The gmsh22 reader reads cells as tuples , instead of CellBlock. This causes issues in determining where to split the data in cell_data_from_raw(). This applies when the number of cells > 1 and cell data is defined. This situation currently does not have a test in the test suite and therefore goes undetected.

This PR fixes the problem and adds tests for this case.

Closes #1257