Identify differential variable for multi-integrals

[AndreG-P]

There is a bug in GenericDifferentialDFixer when multiple integrals are used and both differential variables follow each other. For example

\int \int ... dxdy

Here, dxdy is a single token in the parse tree. Currently we only check for dx or dy as single tokens but did not expect to see multiple dxdy in one. And can become even more complicated with something like

\int \int xy dxdy

Here the entire xy dxdy is a single token.

This must be fixed somehwere in https://github.com/ag-gipp/LaCASt/blob/f996a842f94c540aaca68d9144867ba3d8b81ee3/interpreter.generic/src/main/java/gov/nist/drmf/interpreter/generic/common/GenericDifferentialDFixer.java#L40

[AndreG-P]

Fixed by 23eb4cd