Fix nan initial stress

For non zero load table entries P_factor_HW and P_factor_FW are set to nan, resulting in nan initial stress

sne
NaN	27.4420460148450
NaN	27.4646361264005
NaN	27.4872263762434
NaN	27.5098167667651

load table
time_steps P_steps P_factor_HW P_factor_FW
1955	0	NaN	NaN
1956	-0.124500000000005	0.875100401606393	-0.651485943775096
1957	-0.156080000000003	1.00345976422348	-0.723282931829864