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Convolve fix and test

The ConvolveFourier op was failing with an out of bounds error. The
version of 'FFTConvolution.create' function used seems to require
extended inputs. (From the documentation of the function: "The input as
well as the kernel need to be extended or infinite already as the {@link
Interval} required to perform the Fourier convolution is significantly
bigger than the {@link Interval} provided here.")

There is an alternative version of FFTConvolution.create that takes
non-extended RAIs as input and extends the image using a mirror and the
kernel by zero.

My proposed solution is to:

    Use the alternative version of FFTConvolution.create in the default
ConvolveFourier op.  In the future add a variation of ConvolveFourier
that takes a flag indicating type of boundary extension to use (thus
allowing the caller to choose a different type of boundary such as zero,
exponential fade, etc). In this version of the op pre-extend the inputs
as desired before calling FFTConvolution.create.

I also added a test to run a convolution and check the output.
Merge pull request #33 from bnorthan/convolve_fix

Convolve fix