root/yoda: parse 'value' given as a string of hyphen-separated bin limits

[GraemeWatt]

If there are any non-numeric independent variable values, the current code uses bins of unit width and centred on integers (1, 2, 3, etc.) in the export to ROOT/YODA. There are some records (for example, here or here) where an incorrect YAML encoding has been used such as value: 800 - 1000 or value: '1-1.5' instead of separate low and high values. Such an incorrect YAML encoding gives an acceptable table rendering and is properly handled by the visualisation code, so it could easily be missed in the review process. We should therefore tolerate these incorrect YAML encodings in the export to ROOT/YODA by checking if a string value without explicit low and high values has the format of two numbers separated by a hyphen (with or without surrounding spaces). This case should then be treated in the same way as if separate low and high values were specified.