Removed CellularBase and AT_CellularBase from cellular stack and updated both code and unittests accordingly.
Moved property handling into AT_CellularDevice
New ATHandler functions taken into use for rest of the targets (BG96 was updated initially) to reduce code size. This means basically that new functions using variadic list approach are taken into use and with those one can usually write AT commands in single line instead of multiple lines.
Only internal changes and API's are not modified.
Moved methods to classes CellularDevice and CellularInformation.
SIM interface was removed to simplify cellular usage and
methods better suite new classes.
Updated greentea and unit tests.