bug: `-t`/`--time` option don't accept floating point numbers

cbonsai -l -t0.1 gives error: error: invalid step time: '0.1', which seems like a bug to me, because in the documentation it said that default is 0.03, but setting anything below 1.0 gives an error.

Also it seems a bit strange to me that timeStep is defined as double in struct config, but it then passed to updateScreen which expects float.