Fixing PVTP Writer for arrays

See merge request openfpm/openfpm_io!2

# Conflicts:
#	src/VTKWriter/VTKWriter_grids_util.hpp
#	test_data/vtk_points_pp_header_test.vtp
#	test_data/vtk_points_pp_test.vtp
#	test_data/vtk_points_test.vtp
#	test_data/vtk_points_with_prp_names_test.vtp

Fixed missing consideration of patches that may start / end within a group of nodes to which same pixel-value should be assigned, i.e. patches whose x-dim. is not a multiple of the refinement-factor in x.