Number of nodes used instead of number of vertices

In each of piecewise_constant_mesh and piecewise_constant_field (femtools/Fields_Manipulation.F90) the number of nodes are passed where the number of vertices should be:

- shape = make_element_shape(vertices=old_shape%loc, dim=old_shape%dim, degree=0, quad=old_shape%quadrature)
+ shape = make_element_shape(vertices=old_shape%numbering%vertices, dim=old_shape%dim, degree=0, quad=old_shape%quadrature)

Not a problem as generally this is called for linear meshes.