Write time.Time results as DateTime fields in the spatial writer

[trietmnj]

• Problem: the spatial writer never writes time.Time values. time.Time has reflect kind Struct, which gdalTypes does not map, so the field is created with GDAL's zero type (Integer), and the value switch has no case for it. The lifecycle completion_date column is null in every row (all 437,893 event rows of a DE001 run).
• Fix: create the field as FT_DateTime when the value is a time.Time, and write it with SetFieldDateTime; a zero time stays null. New spatialwriter_test.go writes a GeoPackage and reads the date back.

resultswriters/spatialwriter.go                     completion_date = 2040-11-05 04:17 (time.Time)

 FIRST ROW — create fields
   :74   atype := reflect.TypeOf(result[i])          → Kind() = Struct
   :81   gdaltype := gdalTypes[gotype]               utils.go:11 maps Float32/64, Int32, String only
                                                     → missing key → zero value = FT_Integer
         before:  field "completion"  Integer        ✗
   :82   this PR: time.Time → FT_DateTime            ✓

 EVERY ROW — set values
   :156  switch valType { String, Float32, Float64, Int32, Uint8 }
         before:  Struct falls through, nothing set  ✗  → null
   :171  this PR: case reflect.Struct
   :173           SetFieldDateTime(idx, t)            ✓  (zero time left null)

[HenryGeorgist]

thanks for adding the test.