Fix locate_cell changing coordinates dat version

[ReubenHill]

At present, running this

m = UnitSquareMesh(10, 10)
print(m.coordinates.dat.dat_version)
m.locate_cell([0.5, 0.5])
print(m.coordinates.dat.dat_version)

prints

0
2

This incrementation by two happens in MeshGeometry.locate_cell_and_reference_coordinate when accessing self.coordinates._ctypes

firedrake/firedrake/mesh.py

Line 1892 in 191c339

cell = self._c_locator(tolerance=tolerance)(self.coordinates._ctypes,

The root cause of this is at this line

firedrake/firedrake/function.py

Line 459 in 191c339

c_function.coords = coordinates.dat.data.ctypes.data_as(c_void_p)

and this line

firedrake/firedrake/function.py

Line 459 in 191c339

c_function.coords = coordinates.dat.data.ctypes.data_as(c_void_p)

This PR fixes this by switching to accessing data_ro instead of data.