Model thinning: bug fix – aggressive channel/filter pruning raises an exception

* Fix bug: taking the len() of a zero-dimensional ‘indices’ tensor is not legal.
    Use nelement() instead.
    A zero-dim ‘indices’ tensor occurs when the pruning is very aggressive and
    leaves one channel or filter in the tensor.
* Protect again pruning of all channels/filters of a layer: Raise ValueError if
  trying to create (thru thinning) a Convolution layer with zero channels or filters.
* Tests:
* Some PEP8 cleanup.
* Add some test documentation.
* Refactored some test code to tests/common.py
* Added testing of pruning all the channels/filters in a Convolution