[Fix] z score inplace

[Moritz-Alexander-Kern]

This PR closes #591 .

Changes:

The check for the signal type when using z_score inplace was changed from:

 if inplace and not np.issubdtype(float, sig.dtype): 

to:

if inplace and not np.issubdtype(sig.dtype, np.floating):

Additionally a regression unittest was added.

This was originally discovered in the Elephant_Tutorial_-_LFP_analysis.ipynb notebook, see also https://tutorials.python-elephant.org .

[coveralls]

coverage: 86.64% (-0.3%) from 86.977% when pulling 31e6257 on INM-6:fix/z_score_inplace into 2bd871a on NeuralEnsemble:master.

[Moritz-Alexander-Kern]

build wheels