Patch for failing extraction at volume edges

[McHaillet]

I encountered an error where pytom_extract_candidates.py was crashing because it was trying to extract a particle at the edge of the score volume. This should never happen as in line 46-51 the edge region is masked out.

My fixes:

• Regions of the score map that should not be extracted are now set to 0 instead of -1. This was leading to errors as the peak mask that was multiplied with the score map had 0 values, which lead to -1 values being reset to 0.
• This now also means the extraction cut_off cannot be smaller than 0 and the check for each particle (in extract.py line 92) is now changed to <= instead of < as this guarantees the program stops once a 0 value has been encountered as the maximum.

[sroet]

1 change and please update the version number for this bugfix

[McHaillet]

version is also updated to 0.3.2

[sroet]

LGTM, feel free to merge