Add output validation

[IngaMueller]

in validation.py

add to input_validation: cap_lo <= inst_cap <= cap_up (process, transmission, storage)

add to output_validation:

1. if sto_stored > 0 and sto_retr > 0 --> create warning
2. if transmission line is used bidirectional --> create warning

[lodersky]

For cap_lo <= inst_cap <= cap_up: cap_lo can be bigger than inst_cap to force the model to build new capacities.

Output validation is considered to be developed in the future.