WIP: move field class to new file

__init__.py

lm/nifty_lm.py

lm/nifty_power_conversion_lm.py

nifty_core.py

nifty_field.py

operators/nifty_los.py

operators/nifty_minimization.py

operators/nifty_operators.py

operators/nifty_probing.py

rg/nifty_power_conversion_rg.py

rg/nifty_rg.py

test/test_nifty_field.py

test/test_nifty_spaces.py