Implement MAP to PSI and PSI to inHg formulas

## Context

TODO comments at `MainActivity.java:1048-1049` indicate unit conversion formulas need implementation.

## TODOs

1. MAP to PSI conversion formula
2. PSI to inHg conversion formula

## References

- `MainActivity.java:1048`
- `MainActivity.java:1049`

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*Created from workspace-wide TODO audit.*