Add support for de-referencing of REF_TO datatypes (Note: dereferencing of arrays and structs not yet supported!)

Add support for REF_TO inside arrays and structs (a non-standard feature!)

Code cleanup: with the introduction of anotation_map[], generate_c_vardecl_c can now derive directly from generate_c_base_c

Fix typos in comments

Add comments, and make get_datatype_id_str_c equivalent to get_datatype_id_c

Fix calling of functions whose parameters are of an implicitly declared datatype (currently only makes sense for REF_TO datatypes, but may make sense to other datatypes too if the datatype model is changed in the future).

fix indentations.

in lvalue, add check for REF() fucntion invocation, and the NULL literal

Initialise REF_TO variables to NULL.

Make support for REF_TO ANY a command line option.

Add support for non standard REF_TO ANY (equivalent to void *)

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