Re-added B-spline transformation for non-linear RT alignment in MapAlignerIdentification

Including updated tests.
On one hand, would it do any harm? I currently can't imagine any situation in this context or do you have any specific situation in mind where the `const` would create a problem?

On the other hand it provides a couple of benefits like documentation and semantic annotation that the given method will not perform any changes to the incoming data over. And it avoids some small but problematic to find errors like `=` instead of `==`, passing the variable to a non-const function etc. And it is an additional information for the compiler (e.g., optimisation, inlining, etc.). 

So, IMHO, there is no benefit in removing it.