Reward token generator

Produce _round tokens_ with some lettering that one could use to track time spent on tasks and/or completed tasks. The picture is misleading since you will have to print the box with the [different thing 3136330](https://www.thingiverse.com/thing:3136330)

- Width and height are based on a coin metaphor since people can represent the dimensions and value of a typical coin.
- Tokens can take multiple heights with respect to the reference coin. E.g. writing a page is worth 3x1Euro
- You can add your own (real) coin dimensions or create new ones.
- You can create reward tubes that allow stacking (and visualizing overall progress). See [Thing 3136330](https://www.thingiverse.com/thing:3136330)
- In principle, each different token type should have an other color.

By stacking tokens you can collect rewards. E.g. a tube that contains the equivalent of 50 2EUR pieces will allow spending 100 EUR on anything you like.

It doesn't make much sense to print the included STL example. Th