Implement own version of Duvenauds Grammar

[BracketJohn]

There are some things that occurred during development and evaluation that might be improvements on the grammar of Duvenaud et al.
These improvements also don't seem to be covered in Lloyd et al.

It might be good to implement those in a modified version of duvenauds grammar, some small changes were already made that move away from Duvenauds design, these must then also be moved.

Tasks

• Create new _grammar_duvenaud_modified.py grammar
• Implement/Move over base kernel exclusion
  (rationale: Stops GPs from just taking constant or white as the "best" result)
• Maybe make initial full expansion part of this grammar? Current implementation as part of discover might be unreasonable

TODO: Add more improvements noticed during development here.

[BracketJohn]

One of the biggest reasons to do this currently is the full initial expansion. It seems to be a grammar specific thing atm, but introduces a lot of noise trough out the code.

Another option would also be to just improve this behavior in general.