Performance improvement for compute_enpt2 and add_hci

[peter-reinholdt]

Some work can be avoided when searching for determinants in the compute_enpt2 and add_hci routines.
Rewrite the criterion

$$ \left|H_{ki} c_{i} \right| > \varepsilon $$

as

$$ \left|H_{ki}\right| > \frac{\varepsilon}{\left|c_{i}\right|} $$

The value of the Hamiltonian matrix element is independent of what happens with, e.g., excite_det - so sometimes work can be skipped.

The performance improvement is system-dependent, but here is an example for water/cc-pVDZ.
Timings are in seconds, single thread. The benefit is most substantial when the majority of the determinants are rejected; when most determinants are accepted, there is almost no benefit.

eps1	eps2	Routine	Before	After	Speedup
1.00E-03		add_hci	0.093	0.042	2.23
5.00E-04		add_hci	0.253	0.168	1.51
2.00E-04		add_hci	0.888	0.416	2.14
1.00E-03	0.0001	compute_enpt2	0.237	0.163	1.45
1.00E-03	1.00E-05	compute_enpt2	0.773	0.613	1.26
1.00E-03	1.00E-06	compute_enpt2	2.167	1.913	1.13
5.00E-04	5.00E-05	compute_enpt2	0.720	0.495	1.46
5.00E-04	5.00E-06	compute_enpt2	2.070	1.731	1.20
5.00E-04	5.00E-07	compute_enpt2	8.462	8.005	1.06
2.00E-04	2.00E-05	compute_enpt2	3.097	2.014	1.54
2.00E-04	2.00E-06	compute_enpt2	9.319	7.954	1.17
2.00E-04	2.00E-07	compute_enpt2	46.647	45.395	1.03