Sharpening options in template construction

[cookpa]

This is mostly to address #956, but also adding a few other changes to improve performance.

Users may choose no sharpening, Laplacian (default), or unsharp mask.

Summary statistic: Made mean, median available in antsMultivariateTemplateConstruction.sh

Backwards compatibility is mostly preserved by leaving normalized mean (-a 1) and Laplacian sharpening (-A 1) as the defaults. I did slightly modify how sharpening is used in initial templates:

• Don't sharpen initial templates after population averaging or initial rigid alignment. These initial templates are often very messy and arguably should be smoothed, not sharpened

• In antsMultivariateTemplateConstruction2.sh, use median for initialization - again to help convergence and avoid outliers causing too much trouble.

Misc fixes:

• If -z is specified, ensure that the template exists and don't silently fall back to a population average.

[cookpa]

I need to test antsMultivariateTemplateConstruction.sh on another machine because I'm having trouble with it on my Mac.

[gdevenyi]

Is it worth adding a median method to AverageImages so that the its available elsewhere and a bit cleaner?

[cookpa]

Yeah, updating AverageImages would probably be good. I've been wanting to make an antsAverageImages using the updated CLI, but I have not had time to do that.

[cookpa]

Anyway it looks like the sharpening options are working correctly in both templates, so I'm taking this out of draft mode.