setclone causes crash of QGIS when non PCRaster maps are used

[jvdkwast]

See: jvdkwast/qgis-processing-pcraster#32 (comment)

[OliverSchmitz]

we can improve the error message to state that only PCRaster maps are accepted for setclone
you might be able to circumvent this by
setclone(rows, cols, ...)
readmap(tif)