This patch change the size of real array from 3 to 4 doubles. This patch
remove the length attribute. Now the size computed from the no-zero
value.

There is missing call to the finalize callback in observation system.
This patch fix the problem and provides new observation callback systems
based on function_ref. (Closes: #29).

Adding two global inline function pointers to allocate/deallocate
memory. This patch allows the use of external tools instead of default
malloc/free functions.

The input widget does not affect the default_* variable used to
initialize or reinitialize models. This patch fix the variable.

(closes:#27).

Fix access to the MAXPATHLEN macro

Using std::filesystem::is_* function may throw exception [1].
Use the same functions with the std::error_code parameter to fix
the problem. (closes: #21).

https://developercommunity.visualstudio.com/t/stdfilesystemis-directory-fails-with-exception-if/787098