update Field: new n_components, val_shape attributes, new H1Mixin

- new H1Mixin._setup_shape(), update Field.__init__()
- update Field docstring
inherit H1NodalMixin from H1Mixin, update .set_dofs()
inherit H1HierarchicVolumeField from H1Mixin, update .set_dofs()
remove n_components argument in Variable, FieldVariable constructors

- update Variable.__init__(), FieldVariable.__init__()
- update Variable._setup_dofs() - new n_components, val_shape arguments
- update FieldVariable._set_field() - set n_components, val_shape from field
update sfepy.fem modules for no n_components argument
update scripts, examples and tests for no n_components argument
docs: update for no n_components argument
remove Field.setup_dof_conns() (duplicated in VolumeField.setup_dof_conns())
remove dpn argument of {VolumeField, SurfaceField}.setup_dof_conns()

- update setup_dof_conns()