Add H2 feedin modeling

[fwitte]

Work in progress, still missing:

• separation of stores for CH4 and CH4_H2 mixture
• extra_functionalities
  • ratio of energy transfer from H2 to CH4_H2_mix and resepctive CH4 to CH4_H2_mix buses fixed to max allowable H2 share
  • capacity sharing of feed/return + CH4/CH4_H2_mix pipelines
  • capacity sharing of stores
• ...