Make Index::LabelCompare do numeric comparison on the subscript (numeric) part of an index.

[bimalgaudel]

As of now:
Index{L"i_10"} < Index{L"i_2"} is true.

After:
Index{L"i_10"} > Index{L"i_2"} is true.

While equality (==) (or !=) holds with either string comparison or numeric comparison, ordering/sorting is likely to introduce a bug with string comparison alone, as the programmer naturally might expect a numeric comparison on the numeric portion.

[Krzmbrzl]

The symptoms have been fixed in #177 but imo indices should be refactored to contain a label (string) and an ID (integer) as two separate members. Then comparing the ID numerically is easy.
Where needed the two components could be merged into a single string very easily (easier than it is to disassemble the string into the two components)