Need more developed Internal Euler

[kanav99]

Currently we cannot test some features which require at least one algorithm to run on. For eg. callbacks (brought up in #197). Therefore we need more features in the InternalEuler algorithm.
Maintaining a list of features(more additions are encouraged):

• Callbacks

[ChrisRackauckas-Claude]

Triage: Callbacks are now thoroughly tested via OrdinaryDiffEq and other solver packages. The original motivation (needing InternalEuler for testing callbacks in DiffEqBase) is no longer relevant. Recommend closing.