Added norm to the function reference in the sphinx documentation under Utility functions and conversions.

See #971

coverage: 94.422%. remained the same
when pulling 6828c67 on henriks76:system-norms-doc
into e1e33e4 on python-control:main.

**********************************************************************
File "../control/sysnorm.py", line ?, in default
Failed example:
    ct.norm(Gc, 'inf', tol=1e-11, method='scipy')
Expected:
    1.000000000007276
Got:
    0.999999999992724
*********************************************************************

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Document: generated/control.norm
--------------------------------
**********************************************************************
File "../control/sysnorm.py", line ?, in default
Failed example:
    ct.norm(Gc, 'inf', tol=1e-4, method='scipy')
Expected:
    1.00006103515625
Got:
    0.99993896484375
**********************************************************************
1 items had failures:
   1 of   3 in default
3 tests in 1 items.
2 passed and 1 failed.
***Test Failed*** 1 failures.

You won't get any futher by setting the example to 0.99993896484375. Different platform yield different values. Probably best to round the output:

https://docs.python.org/3/library/doctest.html#warnings

OK will use rounding this time.

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