Allow client-code to treat SatPass as constant

Created by: rwpenney

Various methods within SatPass have changed since gpstk-2.6 which break client code which attempts to treat SatPass as a constant. Query methods like data() force the client to receive a non-constant reference even when they need read-only access to observation data. Various patches have been added to SatPass which improve support for read-only access by clients using "const SatPass" objects. Also, various getter methods have been adjusted to take const std::string& to avoid unnecessary copying of string parameters that are not modified within GPSTk.