FiO2 & flow alarms

[ethanjli]

This PR resolves #323 by having the firmware automatically set FiO2 and flow alarm limits as a tolerance window (+/- 2) around the FiO2 and flow parameters, and filling the remaining gaps for support of FiO2 and flow rate alarms.

Additional minor changes:

• Range protobuf messages now use int32 instead of uint32, to allow for negative numbers in alarm limits and ranges
• The frontend now shows more useful initial values for the FiO2 and flow rate settings on the quick start screen, reducing the number of increment/decrement button-presses needed to adjust to clinically relevant values (in the absence of the rotary encoder).

[ethanjli]

Because this PR has been sufficiently (though lightly) tested on basic functionality and is part of the v0.7 milestone, I'm going to merge this into develop as-is, and as prototype-grade code. For records-keeping:

1. This project is licensed under Apache License v2.0 for any software, and Solderpad Hardware License v2.1 for any hardware - do you agree that your contributions to this project will be under these licenses, too? Yes
2. Were any of these contributions also part of work you did for an employer or a client? No
3. Does this work include, or is it based on, any third-party work which you did not create? No