Immutable Types

[trappmartin]

I should make types immutable by default and add additional mutable types, e.g. MutableSumNode.

Further, I should sub-type Distributions.Distribution instead of my own SPNNode type. This way the code would be compatible with Turing.

• [ ] Make types immutable
• [ ] Add additional mutable types
• [ ] Sub-type Distributions instead of SPNNode
• [ ] Add fit overload for EM fitting of an SPN.