Changed psl interpolation filter

Interpolation filter for psl changed from fixed2interval to tinterp2interval. fixed2interval keeps only the first and the last timestep,
and interpolates the steps between. Therefore it is appropriate only for the fixed variables and those that are used only for the initilazation such as soil moisture and soil temperature.
tinterp2interval keeps all the exixting timesteps, and interpolates those that are missing.

BCtable.MPIESM12LR

@@ -17,7 +17,7 @@ ua       33   109  *         r1i1p1f1 6hr    -    6hrLev  set_hybrid_levels
 va       34   109  *         r1i1p1f1 6hr    -    6hrLev  set_hybrid_levels
 hus      52   109  *         r1i1p1f1 6hr    -    6hrLev  set_hybrid_levels
 ps       1    1    *         r1i1p1f1 6hr    -    6hrLev  
-psl      2    1    *         r1i1p1f1 6hr    -    6hrPlev shift_time(-3hour)|fixed2interval
+psl      2    1    *         r1i1p1f1 6hr    -    6hrPlev shift_time(-3hour)|tinterp2interval
 siconc   31   1    *         r1i1p1f1 day    -    SIday   remapnn|sea_masked|percent2one|tinterp2interval
 tos      37   1    *         r1i1p1f1 3hr    -    3hr     remapnn|sea_masked|celsius2K
 huss     52   105  *         r1i1p1f1 day    -    day     tinterp2interval