work around floating point errors

See merge request !150

allow pyfftw even if MPI is not present

See merge request !144

# Conflicts:
#	test/test_spaces/test_power_space.py

replace ** with clearer operators where possible

See merge request !149

fix package name

See merge request !148

# Conflicts:
#	nifty/field.py
#	nifty/sugar.py

# Conflicts:
#	test/common.py

decompose_power -> keep_phase_information

See merge request !145

Removed duplicates of files and unused reference

See merge request !146

Misc tweaks

See merge request !138