Enhancement: when working with a subgrid, load profile can look a bit weird

[marota]

When the grid is in fact a subgrid with interconnections to other grids, the Import/Export are considered as loads. When accounting for them, this gives sometime a strange Load Profile.
Il would be better to distinguish Import/Export from Grid Load. For that, it might be better to rely on a grid2Op concept of interconnection, not yet implemented.

Example of a weird Load profile for a month of february