starting to implement NEVPT2. Only a stab at it
V(0)_ij,rs sector done.

7 more sectors to go!
clean up (replacing shared_ptr<T>() with nullptr to increase readability). Will be ported to master in the future.
clean up
replication of dfdistt, and some active space matrices
numerator for S(1)ijr is there. The denominator is almost (need to confirm if the second term of (27) is qvec)
clean up
K and K' matrices
implements the S(1)_ij,r sector, verified

6 more sectors to go
some work on RDMs. 2-hole RDM is made simpler
S(2)_ij sector done.

5 more to go
confirmed that the code works also when 3-hole RDM is non-zero
preparing for S(-2)rs
parallel bug fixed. Numerator of S(-2)rs is confirmed.
a bug fixed but still wrong
S(-2)rs done