Dimer::ncore_ redundant and confusing.
Removing nact_ and nvirt_ from Dimer, which were inconsistent and confusing. Now I keep a pair of References (isolated_ref_ and active_ref_) and pull the information from there. I think this will be less prone to error.
removing Dimer::set_coeff()
deoxygenating Dimer class
splitting dimer.cc into more manageable sizes
consolidating functions that prepare DimerCISpace for MEH. Where there was 8, there is now 3 more flexible functions.

TODO: Make a "construct_MEH" function that returns an MEH_base.
implements a construct_MEH function which simplifies building MEH objects and does some related cleanup
minor
Merge branch 'master' of https://github.com/shiozaki/BAGEL