Update GormColumn.groovy

as i understand it, LENGTH_TYPES contains types that do have a length, e.g. VARCHAR(255), but _not_ a scale/precision.

so, if a typeHasLength it should have 0 decimalDigits.

this fixed false positives with dbm-gorm-diff for numeric types with scale for me (postgresql).
Merge pull request #24 from zyro23/patch-1

fixed reverse logic for length_types (not) having decimal digits