Frequently occurring errors

This is a list of badly handled errors in oemof app development as suggested in #514. As we assume that errors we name here are regularly mad, we dubbed them "frequently occurring errors" (FOEs).

AttributeError: 'EnergySystem' object has no attribute 'signals'

oemof-examples#43

ERROR-Optimization failed with status warning and terminal condition infeasible

The energy system (oemof.solph) is designed in a way that there is no solution. Often, there is inflexible (fixed) demand that cannot be fulfilled. Another option is that a fixed (over-) supply cannot go anywhere.

Solving strategies:

1. Add an additional Source that acts as backup/shortage (and does not represent any actual unit of the energy system) - it will make the energy system feasible.
2. To check swiftly whether or not it is the Sink/Source that causes the problem release the boundary condition Flow(fixed=True) and set it fixed=False.