Add an eval block around the dark cal loading

This lets the code proceed despite the Issue #3 errors:
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Tk::Error: PDL: PDL::Ops::assgn(a,b): Parameter 'b'
PDL: Mismatched implicit thread dimension 0: should be 1012, is 14
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which I do not completely understand at this time.
Merge pull request #6 from sot/catch_dark_cal_errors

Add an eval block around the dark cal loading