Read energy totals from network meta data

[maxnutz]

Goal

all pypsa-at networks include the energy_totals data directly in the network-output-file in n.meta.energy_totals. in case this exists, data for energy_totals should be read from there, so there is no extra dependency on another file. If n.meta.energy_totals is not present, read the data from an extra file as is. If this file does not exist, raise an error.

Include all code in one central function get_energy_totals_domestic_share in utils, that reliably returns the domestic to international ratio and cache the energy_totals - data, so that it is available after the first call of this function.

Testing

include a mock energy_totals table to mock_network and check all possiblities:

• network includes energy_totals table -> use it
• network does not include energy_totals table and path to energy_totals file exist -> use energy_totals from file
• network does not include energy_totals table and path to energy_totals file does not exist -> raise an Error.