Research specialization combo multiplier

**What do you want to see?**
The longer you research a tech, the faster research progresses. Switching techs (not just finishing a level) resets the multiplier.

**How would it help players?**
Specialization of empires, to make the game more interesting

**Alternatives**
There are several ways to do it, by points or levels or turns

**Screenshots and mockups**
N/A

**Things to consider**
- Take into account percentage spending. Maybe give a partial or decaying bonus?
- ETA calculation will be messy
- Bonuses need to be moddable or at least disableable.
- A penalty is just a negative bonus, if you want to play that way