Speed up printing JSON to a string. Fixes #377

This was done by reducing the number of string allocations during this process. The string is now allocated in chunks of 1000 characters (which is sort of arbitrary).
Some minor refactoring also.

Author: jacobwilliams

src/json_parameters.F90

@@ -127,6 +127,8 @@ module json_parameters
 
     integer(IK),parameter :: stream_chunk_size = 256_IK !! chunk size for reading stream files
 
+    integer(IK),parameter :: print_str_chunk_size = 1000_IK !! chunk size for writing JSON to a string
+
     integer(IK),parameter :: pushed_char_size = 10_IK !! size for `pushed_char`
                                                       !! array in [[json_core(type)]]