Multiple issue in the pure_python examples

[hcherkaoui]

python2 coreg_demos.py or
python3 coreg_demos.py
'crash' because the SVD do not converge.

python2 pure_python_preproc_demo.py or
python3 pure_python_preproc_demo.py

'crash' because of
Traceback (most recent call last): File "pure_python_preproc_demo.py", line 16, in <module> tsdiffana=True, realign=True, report=True, reslice=True) File "/home/hcherkaoui/src/THESE/pypreprocess/pypreprocess/purepython_preproc_utils.py", line 282, in do_subject_preproc n_sessions = len(subject_data.session_ids) TypeError: object of type 'NoneType' has no len()

in both case.

We should make all examples passed.

[hcherkaoui]

related to #175
But there is more pure Python examples that are broken (than only purepython_preproc_utils.py)