Implemented a new 'param_gnn' called 'transferCapBidirectional', and the associated 'q_bidirectionalTransfer'.

The new parameter allows for defining upper limits for bidirectional transfer, meaning that the sum of the individual (directional) transfers doesn't exceed a set maximum. If provided by data, the new parameter assumes that transfer in both directions is permitted, and fills in any required transfer parameters possibly lacking from data. Data integrity checks are also included to abort model execution and warn user of incompatible parameter values.