Add custom_bmf2bf option to bmmodels

[GidonFrischkorn]

When working with the sdt and m3 models for bmm I thought about a generalizable way to specify when a custom bmf2bf method should be used. The issue is that parts of the bmf2bf.bmmodel method should be executed for all bmmodels but models such as the sdt and m3 need to perform some additional specifications before other parts of the bmmformula should be converted into a brmsformula.

Maybe we can have a chat about this and brainstorm if there are other option. I just wanted to have a decision on this before sending you a pull request with changes that you would not agree with.

[venpopov]

I'm on vacation, will be back next week and we can discuss

[GidonFrischkorn]

I will be on vacation from next ween on and will be back in office the 5th of August. Maybe, we can meet the week I am back from vacation then.

[GidonFrischkorn]

Add bmf2bf.default to bmmformula.R to compile the first line of the brmsformula and have specific methods for bmmodels that require custom compilation of the first formula line.