Integrate opvi (#3)

* add stats layers

* refactor base class

* add stats to init

* fixes in stats layers

* fix findroot

* set better init value

* refactor spec a bit

* change __all__

* delete unused import

* add find root util

* autotestvalue for InputLayer

* add operations over specs

* refactor specs

* add imports

* fix typo

* improve specs

* update travis

* change_version in scrips

* change_version in scrips

* add pytest-cov

* fix python2

* fix typo

* fix tests

* add shape specification interface

* add support for callable shape change

* raise in approx not specified

* change test val

* refactor docstring

* change incoming resolve

* change find root

* more generic layers

* update example

* change default name

* update example

* update example

* fix error message

* update travis, setup and __init__

* travis no cache dir

* change import in helper

* update version and requirements

* update readme

first release