Simplify some objects on creation

[valentinsulzer]

One way to reduce the size of the tree and hence make simplify faster could be to have some simple rules for simplifying objects on creation. For example, Scalar(5) * Scalar(3) returns Scalar(15); similarly for Scalar * Array and Array @ Array.

Since we are now planning on having specific InputParameter objects, this would not cause any problems for parameter estimation. We just lose functionality of being able to update parameter values in an already-created model - so you have to specify up front which parameters you will want to update. This is probably something to do after implementing some kind of input parameter node though, just to be safe.

Thoughts?