Support for storing recurring blocks

In order to be able to replace the old HSD Python implementation in the [SkProgs](https://github.com/dftbplus/skprogs) suite, it would be nice to add support for defining recurring blocks, e.g.
```
$OCCUPATIONS_Ne {
  1S = 1.0 1.0
  2S = 1.0 1.0
  2P = 3.0 3.0
}
```