Fix hang when pressure/gas input is dismissed.

Fixes for iOS14 and iPhone 12

Updated for iOS 14.

Further improvements to mapping adding gas back to volume. Now filling a 80 cuft 3000 psi tank from 0 to 3000 psi with air will show 80 cuft was added and not 79.6 cuft.

- Use real gas law for pricing volume calculations.
- Fix bug where app crashed on high values in working pressure.

- Fixes for iOS10 and 64bit compatible.

- iOS8 support.
- Support pricing for air by volume.
- Minor UI bugs fixed.

- iOS7 UI.
 - Configurable ppO2 END display for mixture selector.

- Fixes hang when editing cylinder name and tapping pressure/mix (Issue #61).
- Minor UI fixes/improvements.

- iPhone5 and iOS6 compatibility.
- Configurable Standard Temperature, no longer locked at 21℃/70℉.
- Configurable list of default gases.
- Print/email bills with custom header.
- Minor internationalisation fixes.

First release.