Allow multiple instances of C++ class API

[Rémi Bèges]

This will require minor changes in C api as well.

Need to define a context structure holding all telemetry-related information. Then define internal methods such as

void _publish_f32(TM_context* context, char * topic, float data)

That will be called internally by

void publish_f32(char * context, float data)

For C api, a single context will be created and existing interface will be kept. For C++, a new context will be instanciated by each new class instance

[Rémi Bèges]

Actually not a true issue. Use case will be very rare, closing for now. Otherwise it will require major modifications of the C++ api and should be done on a per case basis