Fixed a bug that prevented the update of all shared map structures
(ephemeris, iono parameters, etc...)
Re-organizing assistance data input and output

Now we are reading the assistance data from xml files if the correct
configuration option is set; defined the name for the xml files as
const inside control_thread.h; and finally moved the manipulation
of these files to the gnss_sdr_supl_client class, not anymore inside
the control_thread.

Signed-off-by: Javier Arribas <jarribas@cttc.es>
Avoid memory leak

According to the best practices for boost, the shared_ptr constructor
should not be called inside a function call (return in this case).

Ref: http://www.boost.org/doc/libs/1_55_0/libs/smart_ptr/shared_ptr.htm

Signed-off-by: Javier Arribas <jarribas@cttc.es>