HeatPipeline as new custom component

[joroeder]

This PR should implement a new custom component called HeatPipeline.
In #618 there is a short describtion of the aim of the component.

hp = solph.custom.HeatPipeline(
    label='heat_pipe',
    inputs={b_heat_0: solph.Flow()},
    outputs={b_heat_1: solph.Flow(
        # nominal_value=250,
        nominal_value=None,
        investment=solph.Investment(ep_costs=100,
                                    maximum=250),
                                  )},
    heat_loss_factor=0.0002,
    length=200.0,
    conversion_factors={b_heat_1: 0.8}
    )

• Single-In-single-out component
• constant power loss, which is calculated by nominal_value * length * heat_loss_factor
• option of putting conversion factors like regular Transformer

to do

• find right location for the component
• decide on API
• tests
• documentation
• example Heatpipeline example oemof-examples#44

[coveralls]

Coverage remained the same at 94.275% when pulling 4efe40a on features/add_dhs_pipeline_component into 784eabb on dev.

[coveralls]

Coverage remained the same at 94.275% when pulling 4efe40a on features/add_dhs_pipeline_component into 784eabb on dev.

[coveralls]

Coverage remained the same at 94.275% when pulling 4efe40a on features/add_dhs_pipeline_component into 784eabb on dev.

[joroeder]

hey, you will find an example for district-heating network optimization in order to give an idea about what you can use heatpipeline for ...

https://github.com/quarree100/dhs_optimization

It although uses the 'NonConvexInvestmentFlow' of #636
At the moment, there is missing a constant offset not just for costs, but althougt the losses, something like:

P_loss = P_invest * f_loss + f_loss_fix * invest_status

where invest_status is the binary variable of the nonconvex investment.

[codecov]

Codecov Report

Merging #619 into dev will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #619   +/-   ##
=======================================
  Coverage   92.15%   92.15%           
=======================================
  Files          13       13           
  Lines        1377     1377           
  Branches      326      326           
=======================================
  Hits         1269     1269           
  Misses         62       62           
  Partials       46       46           

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