ENH: Array, OptimizerParameters constructors with size and initial value

[N-Dekker]

Added explicit constructors to Array and OptimizerParameters, allowing to specify the size (dimension) and the initial value of each element.

Both std::vector and vnl_vector have a similar constructor.

Included a GoogleTest unit test for OptimizerParameters.

[N-Dekker]

Thanks for your approval, Dženan!

@thewtex Matt, do you think that this pull request could still be included with ITK 5.2? I think it's pretty simple enhancement, but it could certainly make the use of OptimizerParameters easier. This is how I created a const OptimizerParameters variable of two elements for an elastix unit test with ITK 5.1:

const itk::OptimizerParameters<double> optimizerParameters(itk::Array<double>(vnl_vector<double>(2U, testValue)));  

https://github.com/SuperElastix/elastix/blob/b60640fe6e0ef002ca567768cce404573f0c0109/Common/GTesting/elxTransformIOGTest.cxx#L324

The pull request would allow, hopefully for ITK 5.2:

const itk::OptimizerParameters<double> optimizerParameters(2U, testValue);  

[dzenanz]

This does not seem like a potentially problematic feature. #2246 might qualify for that. But we did not announce a feature freeze.

If you are not going to request more reviews, you could merge this PR Niels.

[N-Dekker]

If you are not going to request more reviews, you could merge this PR Niels.

@dzenanz Thanks Dženan, could you please do the merge? I always prefer not to merge my own PRs, if possibly 😃 It looks nicer to me when there are two names with a commit (author + committer).