Add qcd exa to theory.db as theory 798

[niclaurenti]

Adding QCD EXA theory:

• 798 full QCD EXA theory

[niclaurenti]

@andreab1997 @giacomomagni @RoyStegeman you are keeping track of the numbering of the theories more than me. Are 800 and 801 free?
Edit: I mean, they are obviously free in theory.db, I was wondering whether someone has some unpushed theory already called like that

[RoyStegeman]

I think you should be able to use 798 and 799, I'm not aware of anyone else already having reserved theories

[giacomomagni]

I think you should be able to use 798 and 799, I'm not aware of anyone else already having reserved theories

it should be okay, n3lo lasts at 797 and we are currently using 800 for polarized stuff.

[RoyStegeman]

Did you want to merge this?

[niclaurenti]

@RoyStegeman I delete the ''mixed'' theory and then yes