Added support for internal LNA and Attenuator

This version adds functionality to select LNA or Attenuator input stage, check new function SetInputStage().

In previous versions, input stage selection was not included and might cause an amplitude overflow issue when using the internal LNA or attenuator MWSUB3G IoT calibration data.

This new version of RF Explorer library handles amplitude and calibration data correctly.

Support for $z and > 240pts configuration. Tested at 512pts.

Change method changeNumberSteps(). Tested at 512pts in Arduino DUE.
Support for $z - Sweep data
Update library code for new firmware v1.16 of board RF Explorer 3G+ IoT.

Minor change to adapt Seeeduino latest config files

This version fixes a problem when compiling with old GCC as may be the case if using SeeedStudio suggested JSON configuration file for Seeeduino. This problem is not an issue in recent versions of GCC compiler.
Anyway, the updated code works with any version of GCC compiler but uses int16_t type - which may not be as pretty as "short int" from Arduino Wire perspective, but will implement the exact same GCC code.