Feature request: Add modularity to nodes

**Is your feature request related to a problem? Please describe.**
People using modularml might want to use their own implementation of some nodes, we need to support that.

**Describe the solution you'd like**
Make an easy way to switch node constructors or class objects in parser / when creating a a model by hand. 

**Describe alternatives you've considered**
N/A

**Additional context**
N/A
