Two small tweaks to thinning (NOAA-EMC#734)

This PR addresses GSI NOAA-EMC#731. It modifies the satthin.F90 routine to allow
data to be processed unthinned in the GSI without requiring exorbitant
resources and it removes an unnecessary thinning from the read_atms.f90
routine.

The change to satthin reduces the value of `itxmax` from 1.e9 to 1.e7
which should be sufficient for most sensors. But if a larger number is
required for a sensor, this may be passed in via optional argument.

This change will change results for ATMS only as documented in the
issue.

src/gsi/read_atms.f90

@@ -544,11 +544,6 @@ subroutine read_atms(mype,val_tovs,ithin,isfcalc,&
      dlat_earth = dlat_earth*deg2rad
      dlon_earth = dlon_earth*deg2rad   
 
-! Just use every fifth scan position and scanline (and make sure that we have
-! position 48 as we need it for scan bias)
-     if (5*NINT(REAL(IScan(Iob))/5_r_kind) /= IScan(IOb) .OR. &
-          5*NINT(REAL(IFov-3)/5_r_kind) /= IFOV -3 ) CYCLE ObsLoop 
-
 !    Regional case
      if(regional)then
         call tll2xy(dlon_earth,dlat_earth,dlon,dlat,outside)

src/gsi/satthin.F90

@@ -350,7 +350,7 @@ subroutine makegvals
   end subroutine makegvals
 
 
-  subroutine makegrids(rmesh,ithin,n_tbin)
+  subroutine makegrids(rmesh,ithin,n_tbin,itxmax_in)
 !$$$  subprogram documentation block
 !                .      .    .                                       .
 ! subprogram:    makegrids                            
@@ -386,7 +386,8 @@ subroutine makegrids(rmesh,ithin,n_tbin)
 
     real(r_kind)   ,intent(in   ) :: rmesh
     integer(i_kind),intent(in   ) :: ithin
-    integer(i_kind),intent(in   ), optional :: n_tbin 
+    integer(i_kind),intent(in   ), optional :: n_tbin
+    integer(i_kind),intent(in   ), optional :: itxmax_in
     real(r_kind),parameter:: r360 = 360.0_r_kind
     integer(i_kind) i,j
     integer(i_kind) mlonx,mlonj
@@ -402,7 +403,11 @@ subroutine makegrids(rmesh,ithin,n_tbin)
     itx_all=0
     if(abs(rmesh) <= one .or. ithin <= 0)then
       use_all=.true.
-      itxmax=1e9
+      if (present(itxmax_in)) then
+         itxmax = itxmax_in
+      else
+         itxmax = 1e7
+      endif
       allocate(icount(itxmax))
       allocate(score_crit(itxmax))
       do j=1,itxmax