Move filtering code into hera_filters.

[aewallwi]

Move filtering code out of uvtools into the hera_filters repository. hera_filters is at https://github.com/HERA-Team/hera_filters . The reason for this change is that foreground filtering is widely applicable beyond HERA and it makes sense to decouple it from the visualization functionality in uvtools and the associated dependancies. We may want to change the name of uvtools to hera_visualization if we decide to add substantially more plotting functionality in the future.

For now, I've preserved uvtools.dspec and just import hera_filters methods. In a future PR (after people have had time to migrate downstream code) we should probably completely eliminate uvtools.dspec.

[jsdillon]

As I suggested in the other info, can we leave a skeleton here that runs existing functions in hera_filters (without necessarily importing it at the top level)? Like

def some_function(*args, **kwargs):
    from hera_filters import some_function as hf_some_function
    warnings.warn('Deprecation warning: function some_function now lives in hera_filters and will eventually be removed from uvtools')
    return hf_some_function(*args, **kwargs)