Python wrapper: add hint_map to domain decomposition (#827)

- wraps `partition_hint` struct for python
- `partition_hint`struct in `load_balance.hpp` is adjusted in a way that in case of zero unsigned int `cpu_group_size`/`gpu_group_size` it is set to the default value (via setter/getter)
- adds documentation for `partition_hint`  
- adds testing for `domain_decomposition` including `partition_hint` (in unit and unit_distributed)
- adds `partition_hint` in example `ring.py`
- corrects `config.cpp` (and doc) to test for `ARB_GPU_ENABLED` instead of `ARB_WITH_GPU`

Fixes #776 
Addresses #799, #769