Added DIIS convergence acceleration for DMRG-SCF

For test6, the 2-norms of the gradients without DIIS and with DIIS are (the first DIIS vector corresponds to gradient norm 0.00029) :
Iteration     WITHOUT DIIS             WITH DIIS
1             0.516604538370041
2             0.0665091378532402
3             0.00731356385397194
4             0.00124036857578005
5             0.000580917680531611
6             0.000292564558298544     0.000292564703803315
7             0.000145128247244306     0.000145128308544459
8             7.16640164375789e-05     2.47374982300469e-06
9             3.53820878773461e-05     3.47756357543889e-07
10            1.74823208917373e-05     3.7468770018147e-08
11            8.64412171781022e-06     2.21075504619329e-09
12            4.27653281738825e-06
13            2.11642110877153e-06
14            1.04765813161277e-06
15            5.1846095073027e-07
16            2.5667088695156e-07
17            1.27085913044313e-07
18            6.31361443150062e-08
19            3.10080396656071e-08
20            1.5456348422385e-08
21            7.77055778664231e-09