<cmath> declarations and implementations not quite right

[ckormanyos]

The <cmath> function declarations and implementations in the self-written subset of the STL are not quite right.

There needs to be refactoring of global functions for double and to properly distinguish the std functions from the global functions in <math.h>.

In addition, this file at the moment is inconsistent with inclusion of <math.h>. This is not right since it sould be possible to include both <cmath> as well as <math.h> (even if this is not often done in practice).

[ckormanyos]

We could use this issue to also increase the functions to get closer to having all the math functions.