Feature/multi modem support

[parker-hologram]

• Allow setting a specific modem when initializing a network. This can be done by passing the modem into the HologramCloud intializing method for example: HologramCloud({}, authentication_type='totp', network='cellular', modem=modem). Initialize a modem using one of the following methods:
  1. Initialize a modem object with a known good port using a supported modem class in Hologram.Network.Modem for example: EC21(device_name="/dev/ttyUSB4") This initializes a Quectel EC21 on port /dev/ttyUSB4
  2. Scan for all available modems through the new Cellular.scan_for_all_usable_modems() method at Hologram.Network.Cellular. This returns a list of accessible modem objects
• Allow modems to send SMS messages through the modem interface