Add a --reasonable-deafults (or similar) flag

[hanslovsky]

This flag should use reasonable default values for down sampling factors and number of scale levels, and label multiset type precision based on input meta data such as dimensions and resolution. Explicitly set arguments should override defaults.

[hanslovsky]

cc @uschmidt83

[hanslovsky]

This is wrt to the new PainteraConvert class introduced in #33

This could probably be handled with an optional --num-mipmap-levels (or similar) option:

1. If not provided, simply use the scales provided by --scale
2. If provided and smaller than or equal to the number of scales provided with --scale: Fall back to scales provided by --scale
3. If provided and larger than the number of scales provided with --scale (that includes no --scale was provided at all): Fill up the missing scale levels with some heuristic based on the resolution, e.g. get as close to isotropic resolution with anisotropic downsampling factors (twos and ones), then isotropically downsample with factor of 2

This would also need to be considered for container-local and dataset-local options.