More changes for support of semantic verification of il_expressions

Start of support for IL expressions (parenthesis).

Cleaning up the code.

Fix a few more bugs related to FB/function calls in IL.

Semantic verification: Add compatibility check for parameter direction in function invocation.

Add support for semantic verification for implict FB calls in IL (e.g. PV ton_var)

Start of support for semantic verification of FB calls in IL using special IL instructions.

Delete some comments.

Change name help function.

Update comments on general architecture.

Narrow IL code backwords, so as to correctly handle invocations to overloaded functions from IL code.

Code cleanup.

Add printing of semantic errors when calling FB from IL.

Fix typo error in some il visit in fill_candidate_datatypes_c class.

Add semantic verification of FB calls in IL (not yet complete - missing printing of errors!)

Add support for semantic verification of function calls in IL.

Add TODO comment, and delete unused debugging code.

Delete un-used code.

Change negation expression (neg_expression_c) to only allow signed data types.

Move elementary_c::to_string() to datatype_functions.cc/hh

Add, to types symbol table, string datatypes with limited length (my_string_type: STRING[33]) (Thanks Andreas!)

Correctly return the initial value of a string datatype, with limited length (STRING[33] := 'hello')

More detailed error messages for FB invocations in ST.

More detailed error messages in function invocations in ST.

Fix bug in analysing real literals (thanks Andreas!)

Remove debugging messages, and code cleanup.

Better error reporting in function invocations, and in invalid literal values.

Better identification of erros in function/FB invocations.

Print error messages when datatype erros found in ST function/FB calls.

Continue checking data type compatibility inside expressions used to pass paramters to invalid function/FB calls