Improve documentation on secondary sources

[hagenw]

This mentions the Gauss grid for spherical arrays and adds one example plot of a logarithmically spaced linear array. You can preview the rendered page at http://matlab.sfstoolbox.org/en/ssd_doc/secondary-sources/.

In addition, this pull request adds a small comment to the logspread entry in SFS_config.m

[hagenw]

Regarding sphinx, for me it compiles also locally. Try:

virtualenv --python="python3" --no-site-packages "tmp"
source tmp/bin/activate
pip install sphinx
pip install sphinx_rtd_theme
cd doc
sphinx-build -b html -d ./_build/doctrees . ./_build/html-preview/

[fietew]

Sry, misunderstanding. "I did not compile via sphinx"

[hagenw]

OK, then I will clean up the last two commits and rebase the changes.