Now descent minimizer and bugfix

See merge request !97

added all the docstrings for PowerSpace and changed PowerSpace.log to PowerSpace…

See merge request !95

# Conflicts:
#	nifty/spaces/lm_space/lm_space.py
#	nifty/spaces/power_space/power_space.py

Changed RGSpace fft smoothing kernel to be Gaussian, added some

See merge request !91

# Conflicts:
#	nifty/spaces/rg_space/rg_space.py

corrected the docstrings to respect inhereted docstrings and added docsgtrings for all methods where they were missing

added all the docstrings for PowerSpace and changed PowerSpace.log to PowerSpace.logarithmic as well as PowerSpace.harmonic_domain to harmonic_partner

Docstrings

be more concise about adjoint_inverse and inverse_adjoint

See merge request !90

Document FFTOperator and rename its "inverse_times" to "adjoint_times"

See merge request !84

renamed QuasiNewtonMinimizer to DescentMinimizer, removed position.setter from energy and corrected step_length in DescentMimimizer

LinearOperator: better fallback strategy if a specific operation is not defined
test_misc: test condition on scalar products under FFTOperator

remove 'implemented' keyword

See merge request !85