[BUG] Grid reinforcement using only 10% criterion not correct

[birgits]

Describe the bug
The combined_analysis setting in the grid reinforcement is not correctly implemented, as it will solve voltage problems solely through LV grid expansion, which might in some cases not be sufficient. Also, as long as the n-1 security is not checked correctly (see #202), this will lead to underestimated grid expansion needs, as there could be high voltage drops in LV grids further upstream that are only detected in the fault case when they are served from the other side of the ring.
I would suggest to remove the combined analysis for now and maybe reimplement it once #202 is fixed.

[birgits]

Currently, a warning is raised when split_voltage_band is set to False, but it should still be fixed at some point.