rewrite plot_latencies, update compute_times

[jelmerborst]

• rewritten plot_latencies to really handle bar and line plots. I propose to remove plot_latencies_average.
• added the ability to use hmp estimates as input to plot_latencies. If agreed, I'd argue for just removing the times-as-array option to clean up the code.
• added the ability to plot condition fits automatically with plot_latencies
• extended compute_times to be able to deal with missing stages when calculating durations
• extended compute_times to return time in ms instead of samples, also made this the default for plot_latencies. However, if time_step is provided, it takes precedence.

Still missing:

• error bars when using an hmp object (error calculation will be added to compute times)

[jelmerborst]