Move Space classes in separate files

__init__.py

demos/demo_excaliwir.py

demos/demo_faraday.py

demos/demo_tomography.py

demos/demo_wf1.py

demos/demo_wf2.py

demos/demo_wf3.py

lm/__init__.py

lm/gl_space.py

lm/hp_space.py

lm/lm_space.py

power/power_space.py

power/rg_power_space.py

rg/__init__.py

rg/nifty_rg.py

test/test_nifty_fft.py

test/test_nifty_field.py

test/test_nifty_operators.py

test/test_nifty_spaces.py