Fix memory leak in write str attribute

Partially releated to #618 (closed).

I found, that we have memory leak when writing the scalar string attribute due to we allocate the new char* memory here and then cpptango copies it here and nobody releases the allocated memory. So I just added a simple fix to our _fill_scalar_attribute<Tango::DEV_STRING> method.

Unfortunately, we have to copy data twice, due to cpptango accepts C-style strings (null-terminated char*). So we first have to create such an object from PyObject, and then cpptango copies it for the second time. Stupid...

Additionally I did some clean-up from "pre-obligatory-numpy" era to improve code readability