Add `featureTemplate` as a read-only field in class `GDALVector`

[ctoney]

Read-only fields

$featureTemplate

' A list of the attribute and geometry field names, with NA values equivalent

' to OGR NULL values. The list elements are fully typed with the corresponding

' missing value types assigned (NA_integer_, NA_real_, NA_character_,

' etc.). The featureTemplate is useful to initialize a new empty feature,

' to which field and geometry values can be assigned, for use with the

' $createFeature() method (create and write a new feature within the layer).

' Note that geometry fields are initialized as character type in the

' template, but may be set either to a character string specifying a

' geometry in WKT format, or to a raw vector containing a geometry as WKB.