Fix for disappearing Around

[maxitg]

Changes

• Due to a weed in Wolfram Language, MeanAround sometimes returns a single number instead of Around:

  In[] := MeanAround[{1, 1}]
  Out[] = 1

• This sometimes breaks ./performanceTest.wls, which assumes Around objects.

• This PR implements a workaround for this case.

Examples

• This case happens rarely, but manually assigning magnitude = 1 in differenceString does work:

$ ./performanceTest.wls master HEAD 2
Testing 811551cb72de921652430003f56fcf25e4a8f185

Testing 19264e2884d6d3cc6831c1fcce2941225f7d217c

Single-input rule                       1 ± 0 %
Medium rule                             1 ± 0 %
Sequential rule                         1 ± 0 %
Large rule                              1 ± 0 %
Exponential-match-count rule            1 ± 0 %
CA emulator                             1 ± 0 %
Multiset addition                       1 ± 0 %

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This change is 

[maxitg]

Another issue could appear if the old timing is identically zero, so I added that case as well.