_mass2tof returns negative time of flight values for extremely low masses

[alex-l-kong]

Describe the bug

Certain users have started to specify panels with mass ranges 15 or lower. However, the _mass2tof function will return a negative time of flight value for these.

Because we convert the returned TOF array of _mass2tof to np.uint16, this causes the final range passed into the corresponding _extract_bin.pyx function to be incorrect, because any negative values get wrapped around to 65536 + (negative TOF value). Additionally, these TOF ranges will be incredibly large, and if it is part of a lowest-highest TOF range specification, it will cause memory issues.

Expected behavior

We'll need to discuss further, I'm not sure what the explanation of a negative TOF value should be.

To Reproduce

Run the function as is, and specify a mass in the provided panel with "Stop" equal to 15 or lower.