oemof/oemof-solph: Capable Custom Components

New features

• Custom component: oemof.solph.custom.PiecewiseLinearTransformer. A transformer model with one input and one output and an arbitrary piecewise linear conversion function. On how to use the component, refer to the test script <https://github.com/oemof/oemof-solph/blob/dev/tests/test_scripts/test_solph/test_piecewiselineartransformer/test_piecewiselineartransformer.py>_ and example <https://github.com/oemof/oemof-examples/blob/master/oemof_examples/oemof.solph/v0.4.x/piecewise/piecewise_linear_transformer.py>_.
• Enhanced custom SinkDSM:
  • Renamed keyword argument method to approach
  • Renamed approaches interval to oemof and delay to DIW
  • Added modeling approach DLR (PhD thesis of Hans Christian Gils 2015)
  • Included load shedding
  • Introduced recovery_time in DIW approach
  • Introduced shift_time and other parameters for DLR approach
  • Included investments in DSM

• Allow conversion factor of zero for GenericTransformer
• Check number of Flows in GenericStorage
• Split code into submodules