Better knowledge of the opmode

[tperale]

• In which mode should the SX127X be running to change the channel ?
• In which mode should we go after completed transmission (currently standby

  sx127x/src/sx1272.c

  Line 207 in b74c17f

  sx127x_set_opmode(&SX1272_DEV, sx1272_mode_standby);

  ) ?
• Is the module settled to a default mode after TX completion ?
• Measure time to switch from sleep mode to standby and standby to sleep mode.
• Can we go in sleep mode instead of standby when turning off the radio.