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.

Update library code for new firmware v1.16

Updated library code to support firmware v1.16 for board RF Explorer 3G+ IoT.
Updated library function changeNumberSteps(). Tested and supported with 512 sweep points in Arduino DUE.
Include support for extended $z sweep mode with up to 65K data points.

Updated Git Repository structure with independent sub-modules.

Project RF Explorer 3GP IoT added to the library manager in Arduino IDE v1.8.1

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.