Remove oo_.dr.{nstatic,npred,nboth,nfwrd,nspred,nsfwrd}

Replace them by equivalents in M_ (and an extra one: M_.dynamic).

IMPORTANT POINT: oo_.dr.npred used to count both purely backward and mixed/both
variables. This was the cause of lots of confusion. The new M_.npred only
counts purely backward variables.

We now have the following indentities:

M_.npred + M_.nboth + M_.nfwrd + M_.nstatic = M_.endo_nbr
M_.nspred = M_.npred + M_.nboth
M_.nsfwrd = M_.nfwrd + M_.nboth
M_.ndynamic = M_.npred + M_.nboth + M_.nfwrd