Feature request: Computations on ranges

Sometimes it's useful to have support for computations based on ranges, especially for value estimation. The core idea is like [Unsure Calc](https://github.com/filiph/unsure) ``` 1400~1700 * 0.55~0.65 - 600~700 - 100~200 - 30 - 20 = -60~220 ``` Having that with all the Numbat power especially with the units and dimensions would come really useful.