Add a public wrapper function for free()

[derselbst]

to allow proper deallocation for programming languages other than C/C++.

[jjceresa]

May be fluid_free() could be renamed fluid_free_mem(), to avoid confusion with other delete_fluid_**() function ?

[derselbst]

to avoid confusion with other delete_fluid_**() function ?

That's why I've put an explicit comment for that to fluid_free(). I would state that fluid_free_mem() only makes naming unnecessarily complicated.

[jjceresa]

Oh yes, the comment (i had not read before!) is clear, so fluid_free() name is fine.