Add type annotations to _step module

Additionally, Field and Rprof are now NamedTuple instead of
nametuple instances.

Author: amorison

docs/sources/apiref/step.rst

@@ -4,4 +4,8 @@ _step
 .. automodule:: stagpy._step
    :members:
    :private-members:
-   :exclude-members: _init_shape, _get_raw_data, _scale_radius_mo
+   :exclude-members: _init_shape, _get_raw_data, _scale_radius_mo,
+                     Field, Rprof, _present_fields
+
+   .. autoclass:: Field
+   .. autoclass:: Rprof