fix(train-cloud-micro): inquire about network file

This commit replaces an `open` statement with an `inquire`
statatement to prevent a crash when mistakenly attempting
to reopen an already open file.

demo/app/train-cloud-microphysics.F90

@@ -261,10 +261,14 @@ subroutine read_train_write(training_configuration, args, plot_file)
       ), &
       network_file => args%base_name // "_network.json" &
     )
-      open(newunit=network_unit, file=network_file, form='formatted', status='old', iostat=io_status, action='read')
-
+      check_for_network_file: &
+      block
+        logical preexisting_network_file
+
+        inquire(file=network_file, exist=preexisting_network_file)
+
       read_or_initialize_network:   &
-      if (io_status==0) then
+      if (preexisting_network_file) then
         print *,"Reading network from file " // network_file
         trainable_network = trainable_network_t(neural_network_t(file_t(string_t(network_file))))
         close(network_unit)
@@ -303,6 +307,8 @@ subroutine read_train_write(training_configuration, args, plot_file)
 
       end if read_or_initialize_network
 
+      end block  check_for_network_file
+
       print *, "Conditionally sampling for a flat distribution of output values"
 
       flatten_histogram: &