Append to shapefiles if they exist

Instead of creating them. Motivation: avoid time spent concatenating
shapefiles in `inst_eddies.py`. This concatenation time, on Ciclad, is
of the same order of magnitude as the time spent by the Fortran
executable.

Inst_eddies/CMakeLists.txt

@@ -7,7 +7,8 @@ add_executable(inst_eddies inst_eddies.f90
   ${PROJECT_SOURCE_DIR}/Common/derived_types.f90
   ${PROJECT_SOURCE_DIR}/Common/shpc_create.f90 nearby_extr.f90
   get_var.f90 ${PROJECT_SOURCE_DIR}/Common/shpc_close.f90
-  write_aux.f90 config.f90)
+  write_aux.f90 config.f90 ${PROJECT_SOURCE_DIR}/Common/shpc_open.F90
+  ${PROJECT_SOURCE_DIR}/Common/read_field_indices.F90)
 
 target_include_directories(inst_eddies PRIVATE ${fortrangis_INCLUDE_DIR})
 

Inst_eddies/inst_eddies.f90

@@ -18,6 +18,7 @@ program inst_eddies
   use set_max_speed_m, only: set_max_speed
   use shpc_close_m, only: shpc_close
   use shpc_create_m, only: shpc_create
+  use shpc_open_m, only: shpc_open
   use write_aux_m, only: write_aux
   use write_eddy_m, only: write_eddy
 
@@ -67,6 +68,7 @@ program inst_eddies
   ! target extremum
 
   character(len = 30) time_unit
+  logical exist
 
   !--------------------------------------------------------------
 
@@ -177,8 +179,16 @@ program inst_eddies
 
   ! Write snapshot:
 
-  call shpc_create(hshp_cyclo, shpc_dir = "SHPC_cyclo", cyclone = .true.)
-  call shpc_create(hshp_anti, shpc_dir = "SHPC_anti", cyclone = .false.)
+  inquire(file = "SHPC_cyclo/extremum.shp", exist = exist)
+  
+  if (exist) then
+     call shpc_open(hshp_cyclo, shpc_dir = "SHPC_cyclo", rank = 0)
+     call shpc_open(hshp_anti, shpc_dir = "SHPC_anti", rank = 0)
+  else
+     call shpc_create(hshp_cyclo, shpc_dir = "SHPC_cyclo", cyclone = .true.)
+     call shpc_create(hshp_anti, shpc_dir = "SHPC_anti", cyclone = .false.)
+  end if
+
   n_cyclo = 0
   n_anti = 0