getVertexPositions as template function

[brezhnyev]

[nmwsharp]

Hi! Thank you for the pull request.

This seems reasonable, though can you add a default version so that existing code that just calls getVertexPositions() still works? Perhaps something like:

std::vector<std::array<double, 3>> getVertexPositions(
    const std::string& vertexElementName = "vertex") {

    return getVertexPositions<double>(vertexElementName);

}

(I feel like there ought to be some way to do this with a typedef/using, but I don't know what it is)

This should also get the CI test above to pass.