Burn-in length stuck at 0.25 on saved slicecore objects

[amandakaris]

The issue:
The burn-in on saved slicecore objects is automatically set to 0.25 even if, when initially creating the core, I set the burn-in length to be something different. Specifying a burn-in length when loading in an old slicecore does not change it from 0.25.

My quick fix:
Change line 79 in slices.py to super().__init__(corepath=corepath, burn=burn) so that specifying the burn-in length when loading an old slicecore replaces the default 0.25.