generated filename for outputs
Previously they were just echoing the input parameter value,
which isn't always a string and accordingly was broken for
temporary outputs. This caused models to break when the output
of a GDAL algorithm was used in a subsequent model step.
Also:
Thanks to @agiudiceandrea:
Fixes#19900
Statistics were erroneously computed on the dissolve field, rather than on the statistic attribute field as it should be.
Fixes#19307
Handle filenames with white spaces (in the same way as Buffer, OffsetCurve, OneSideBuffer, PintsAlongLines algs).
Include descriptive text with the specified parameter value
in error, and always check that sources were loaded to avoid
raw Python exceptions when they are not
