removing zeros in profile data caused by Thermo bug

Some Thermo QExactive profile data contain zero intensity data points that are clearly not part of the profile. The example below shows two peaks in profile mode. Data points are roughly 0.006 Th apart. Right after the 4th and 5th data point, the spectrum contains two zero-intensity data points (green +). They are clearly artefacts and not part of the profile.

This pull request removes these zeros in the SplineSpectrum constructor and thereby ensures correct interpolation of the profile data.

![thermobugzeros](https://cloud.githubusercontent.com/assets/6149805/4941281/6ac8f932-65e1-11e4-8725-54ca7fabcf9f.png)

maybe document the difference between mz_slim1 and mz_slim2