Merge branch 'sphinx_docs' into 'master'

sphinx/Makefile

sphinx/source/_static/nifty_logo_black.png

sphinx/source/_static/nifty_logo_white.png

sphinx/source/composed_operator.rst

sphinx/source/conf.py

sphinx/source/conjugate_gradient.rst

sphinx/source/descent_minimizer.rst

sphinx/source/diagonal_operator.rst

sphinx/source/endomorphic_operator.rst

sphinx/source/fft_operator.rst

sphinx/source/field.rst

sphinx/source/gl_space.rst

sphinx/source/gllm_transformation.rst

sphinx/source/hp_space.rst

sphinx/source/hplm_transformation.rst

sphinx/source/index.rst

sphinx/source/invertible_operator_mixin.rst

sphinx/source/lm_space.rst

sphinx/source/lmgl_transformation.rst

sphinx/source/lmhp_transformation.rst