Allow python section.disconnect().

[nrnhines]

Raise error if arg given to the hoc functions disconnect or
delete_section.

There are many such functions. Should this PR raise errors for those as well?
@ramcdougal , I thought you did a bunch of these but I can't find the changeset.

Closes #1447

A test of this has been added to test_basic.py and prints:

CHECKING: h.disconnect(sl[2])
NEURON: disconnect takes no positional arguments and disconnects the HOC currently accessed section. If using Python, did you mean a named arg of the form sec=section? Or you can use section.disconnect().
...
CHECKING: h.delete_section(sl[2])
NEURON: delete_section takes no positional arguments and deletes the HOC currently accessed section. If using Python, did you mean a named arg of the form, sec=section?

I think most of the offending functions could use one or the other of those styles, so they could become function calls that check if an arg exists and raise an error with the name substituted in the two or three places.

[ramcdougal]

I'd argue that any systematic change to check for the wrong number of arguments should be in a separate pull request. The main feature here is a new section method...

I think in the past I've made one off changes to raise an error on a specific function; nothing systematic. You could reasonably do that here if this pull request was about improving section disconnect.

[ramcdougal]

Looks good to me.

Stylistically, I wouldn't have created a variable x in the test because I dislike single-variable names because it gives me no intuition about why x == i but that's just me.

[Helveg]

Current set of changes looks good, clear error message.

Some small comments in the tests about what is being tested might be useful. Like "Check that disconnected section has 2-piece subtree"