rename NBIO occurences to Cellular

fix modem validity check

replaced calls to machine.idle with utime.sleep_ms

Update README with updated examples

update DNS-Client version and update dns example to have better error detection

changed the detection of connection status plus some refactorings

created list of streams and dynamically assign to new socket the call of getaddrinfo

updated README file

Updated all examples to align with the code changes

Major refactoring to better align to usocket API

moved dns_query folder from the examples to main folder as it will be used in the core code.

Added README file for examples folder noting the requirement of external libraries as git submodules

Deactivating timeout argument in send_at_cmd as it is not supported by the default Pycom firmware

IPv6 DNS resolve example using library DNS-Client

added DNS-Client as submodule used in DNS resolve test