parsing of `X..Y` ranges for LIST keyword fails

[jmbuhr]

I am not sure how often this is used for other places in the input file, but e.g. the LIST keyword of ISOLATED_ATOMS.
takes a range of the form start..end with integers start and end.
However, right now this is coded as integer in https://github.com/cp2k/cp2k-input-tools/blob/develop/cp2k_input_tools/cp2k_input.xml
(line 429209). This errors out when parsing.

A temporary fix is to change it to string, though this does not give us structured data to work with.