Update some comments. (No changes to code)

Introduce absyntax class for keyword ANY, and use REF_TO(ANY) as datatype of NULL literal.

Fix bug (introduced a few commits ago) when generating SFC actions that are mapped onto variables --> Make generate_sfcdecl_c stateless (i.e. does not depend on order by which it is called).

Add support for the 'NULL' keyword, defined in version 3 of IEC 61131-3.

for option '-Op' (generate <pouname>.h/c): add '#include <pouname>.h' to the .c file, and '#ifdef <pouname>_H' to the .h file

Ass include of iec_std_lib.h in POUS.h file.

Add support for implicitly declared REF_TO datatypes.

With -Op option --> Place implicitly defined datatypes in the .h file corresponding to the POU where they are used.

Add name of POU as prefix to names of implicitly defined datatypes

Implicitly defined array datatypes use new naming method (greatly simplifies stage 4 code).

array_specification_c may be used as a datatype! Add it to spec_init_separator_c

Change REF() operator to return the correct REF_TO datatype datatype.

Narrow array subscripts correctly, even in the presence of other datatype errors (so we do not generate error messages for array subscripts that do not contain errors).

Add support for REF_TO derived datatypes (along with datatype verification)

Add option to place each POU in a distinct .c and .h file pair.

code re-organisation. Preparing for the changes to come in the next commit (one C file for each POU)