Option to turn off logging

The best way to suppress logging is to set calc.log = Logger(None). However, the logger is set during the call to Amp.set_label, which automatically prints the header as well. We should have a way to suppress logging (e.g., when Amp is used for energy / force calls writing to disk slows things down). It is surprisingly convoluted how it is set, so turning it off was not straightforward.