Internal: Update diff logic to handle CSV files that have rounding differences

[georgemccabe]

The s2s_mjo:4 use case produces CSV files that have rounding differences in the values.

Describe the Enhancement

Update the diff logic in metplus/util/diff_util.py to handle this situation and diff the numeric values using 6 digits of precision.

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[georgemccabe]

This is a duplicate of #1874, which was completed with PR #1875