[SolutionArray] Add array size methods

[bryanwweber]

Changes proposed in this pull request

• Add a few methods to SolutionArray to have it better match the NumPy API.

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• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
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[ischoegl]

Nice addition! But I do have a question about the documenting style (other than the apparent typing issue, which should be easily resolved)

[bryanwweber]

@speth @ischoegl The reason the tests are failing for Python 3.7 is that math.prod was added in 3.8, see: https://docs.python.org/3/library/math.html#math.prod

I suppose I'll switch it to NumPy. The reason I chose math.prod was that the NumPy docs for ndarray.size specify that a standard arbitrary size Python int is returned from ndarray.size, similar to the result from math.prod, as opposed to np.prod that returns an np.int_ type. It seems unlikely that this will be a problem, but when trying to match an API, I think it's useful to match it as closely as possible.

The real question here is, is this worth dropping 3.7 support over? Thoughts are welcome 😄

Edit: Although, the math docs say that the functions return a float, unless otherwise specified, so this may not be as meaningful as I thought. I'll just switch to NumPy, never mind!

[ischoegl]

This all looks good to me.