Add ability for user to change valid temperature bounds

[adenoz]

RusQTTbom currently has hard coded bounds for what a valid temperature value looks like. These are as follows:

• min -20
• max 50

In colder areas of Australia (noting the BOM weather only covers Australia), there may be valid colder days than -20. Likewise, some areas may want to increase the lower bound as the temperate never even gets to freezing.

This is a minor extra data validation step ensuring only as correct data as possible is being published.