Add simplified variant for H2 feed-in restrictions into CH4 grid

[fwitte]

The H2 feed-in restrictions will be simplified within the eGon-data workflow by assigning a p_nom value, that is a fraction of the CH4 transport capacity of the connected pipelines. The modeling of H2 feed-in restrictions proposed in eTraGo (see openego/eTraGo#392) are quite complex and there should be a more simple approach available.