This calculates the entropy for the image, based on the histogram.
Because this uses image histogram data directly, the existing C
function underpinning the `image.histogram()` method was abstracted
into a macro, and a new C function was added that uses this macro.
The new `image.entropy()` method is based on `image.histogram()`,
and will accept the same arguments to calculate the histogram
data it will use to assess the entropy of the image.
The algorithm and methodology is based on existing Python code:
    https://git.io/fhmIU
... A test case in the `Tests/` directory, and doctest lines in
`selftest.py`, have both been added and checked.

Subsequent commits:
* Using assertAlmostEqual() in entropy tests
* Added description of `extrema` arguments.
* Only test seven digits of float returned by im.entropy()

Removed log2