Turning off decoherence correction

[JanosJiri]

The decoherence correction is turned off by setting decoh_alpha=0. However, this value corresponds rather to an infinitely strong decoherence correction. Turning it off means setting it to infinity. This is quite a confusing feature. We should create a new keyword for turning on/off the decoherence correction. When someone uses decoh_alpha=0, ABIN should issue an error since this would lead to a divergence.