Add flow test for failiure in elastic flow

Author: gpetretto

tests/test_data/vasp/Si_elastic_fail/elastic_relax_2_6/inputs/INCAR

@@ -0,0 +1,24 @@
+ALGO = Normal
+EDIFF = 1e-07
+EDIFFG = -0.001
+ENAUG = 1360
+ENCUT = 300
+GGA = Ps
+IBRION = 2
+ISIF = 2
+ISMEAR = -5
+ISPIN = 2
+LAECHG = False
+LASPH = True
+LCHARG = False
+LELF = False
+LMIXTAU = True
+LORBIT = 11
+LREAL = False
+LVTOT = True
+LWAVE = False
+MAGMOM = 8*-0.0
+NELM = 200
+NSW = 99
+PREC = Accurate
+SIGMA = 0.2

tests/test_data/vasp/Si_elastic_fail/elastic_relax_2_6/inputs/KPOINTS

@@ -0,0 +1,4 @@
+pymatgen with grid density = 100 / number of atoms
+0
+Gamma
+2 2 2

tests/test_data/vasp/Si_elastic_fail/elastic_relax_2_6/inputs/POSCAR

@@ -0,0 +1,16 @@
+Si8
+1.0
+   5.4229442218237667    0.0000000000000000    0.0000000000000003
+  -0.0000000000000003    5.4502640129743263    0.0000000000000003
+   0.0000000000000000    0.0000000000000000    5.4502640129743263
+Si
+8
+direct
+   0.2500000000000000    0.2500000000000000    0.7500000000000000 Si
+   0.5000000000000000    0.0000000000000000    0.0000000000000000 Si
+   0.2500000000000000    0.7500000000000000    0.2500000000000000 Si
+   0.5000000000000000    0.5000000000000000    0.5000000000000000 Si
+   0.7500000000000000    0.2500000000000000    0.2500000000000000 Si
+   0.0000000000000000    0.0000000000000000    0.5000000000000000 Si
+   0.7500000000000000    0.7500000000000000    0.7500000000000000 Si
+   0.0000000000000000    0.5000000000000000    0.0000000000000000 Si

tests/test_data/vasp/Si_elastic_fail/elastic_relax_2_6/inputs/POTCAR.spec

@@ -0,0 +1 @@
+Si