Add parameter scalePeaks to plotSpectraMirror

[jorainer]

This PR adds a parameter scalePeaks to plotSpectraMirror,MatchedSpectra that allows to scale the peak intensities before plotting.

[philouail]

This looks all good to me, and definitely necessary to prevent extra code, but is there a reason why the default is not "TRUE" ? From what i've seen it's pretty necessary all the time and for example in matchms it is one of the "basic" filtering.

[jorainer]

thanks for the review @philouail - the reason to have the default scalePeaks = FALSE to not change the default behavior of the function. So, people that used the function before would still get the same results. Some might be worried/confused if the results changed from one version to the next.