Uplink Payload Formatter: Implement handling of invalid data

[matthias-bs]

Currently, invalid signals transmitted in the uplink message are decoded like any other data.
PayloadBresser.cpp now uses special values to indicate invalid data:

// Encoding of invalid values
#define INV_FLOAT 0x3FFFFFFF 
#define INV_UINT32 0xFFFFFFFF
#define INV_UINT16 0xFFFF
#define INV_UINT8 0xFF
#define INV_TEMP 327.67

The Uplink Payload Formatter shall omit output data in the JSON message for any input data encoded as invalid.