Fix/docstring warnings

[Moritz-Alexander-Kern]

This PR fixes mostly indentation errors in several doc-strings. The content of the documentation is not affected.

Before:
when building the documentation with:

cd doc && make html

miscellaneous warnings appeared, most of them indentation errors. 51 warnings
See log of make html: Warnings_log_8_make html.txt

With this PR:

• fixed indentations
  To fix warnings like:

elephant.asset.asset.ASSET.intersection_matrix:28: WARNING: Block quote ends without a blank line; unexpected unindent.

• added new template for sphinx extension autosummary to document the classes of spike_train_generation.
  This fixes:

WARNING: autosummary: failed to import NonStationaryGammaProcess.process_operational_time.
2022-04-14T17:25:55.5004709Z Possible hints:
2022-04-14T17:25:55.5005191Z * ModuleNotFoundError: No module named 'NonStationaryGammaProcess'
2022-04-14T17:25:55.5005828Z * AttributeError: type object 'NonStationaryGammaProcess' has no attribute 'NonStationaryGammaProcess'
2022-04-14T17:25:55.5006343Z * KeyError: 'NonStationaryGammaProcess'
2022-04-14T17:25:55.5006907Z * AttributeError: type object 'NonStationaryGammaProcess' has no attribute 'process_operational_time'

• added missing "`" to fix warnings like:

docstring of elephant.conversion.BinnedSpikeTrain:94: WARNING: Inline interpreted text or phrase reference start-string without end-string.

• fixed citation not found in:

docstring of elephant.spike_train_correlation.cross_correlation_histogram:69: WARNING: citation not found: correlation-Eggermont2010_77

Only 1 Warning (of 51) remains:
DeprecationWarning: pandas_bridge module will be removed in Elephant v0.8.x

See log: Warnings_log_8_make html_after_fix.txt

[coveralls]

Coverage: 87.457% (-0.04%) from 87.493% when pulling 1932770 on INM-6:fix/docstring_warnings into eb9f00e on NeuralEnsemble:master.

[Moritz-Alexander-Kern]

build wheels

[Moritz-Alexander-Kern]

build wheels