Cleaning TODO.
Taken from Blas' v1.5 checklist and revised for v2 by Claudio and for v3 by Popa Adrian.

The order is not significant.

Macros
M1.- Remove NOT_USED_OR_REPLACED Macro
Pending.

M2.- Remove BOOLEAN, TRUE and FALSE macros and replace with keywords.
Done where except some places where it doesn't make sense to remove them.

M4.- Remove TEXT and SCHAR macros.
Tricky and in some places SCHAR is really used as signed char.

Code
C1.- Remove clang++ warnings (excluding parentheses, unused var, non complete switches and pragmas)
Pending.

C2.- Define (type-safe?) unions for nod_arg and lls_object. 
	Nod_arg is used in several places of the code to contain pointers to several kinds of objects or numbers, whether is defined as pointer to xxx_nod.
	The same happends with lls_object in lls (linked list) where the lls_object can store pointers or numbers, whether is defined as pointer to xxx_nod.
	See gpre.h nod_arg in gpre_nod struct and lls_object in lls struct.
Another solution exists.

C3.- Remove casts.
Partially done.

C4.- Replace macros with code with inline functions.
Partially done.

Builds
B1.- Posix take cpp generated from epp out of src directory.
Unknown state.