Fixes #177 .

I think this aligns the behavior a lot better across the different file formats (see the test sets in test/test_tables.jl)

This behavior is implemented through `getfield(feature, i)` which makes use of `getfieldtype(feature, i)`. That way, we are aligned in behavior for field subtypes even for e.g. displaying the fields.

Because a lot hinges on distinguishing between whether a field is set versus whether it is null (see Toblerity/Fiona#460 (comment)), I have also added support for `isfieldnull()`, `isfieldsetandnotnull()`, and `setfieldnull!()`.

As mathieu has observed, `OGRUnsetMarker` and `OGRNullMarkerare` are mutually exclusive. We implement that case to return nothing if it ever comes up, but it is not possible for us to the corresponding code path.

Because getdefault() is meant to be used only when setting fields for notnullable columns with missing values, we make it return `nothing` instead of `missing` to unset the field.

isfieldsetandnotnull() is just a convenience function.

…fieldtype when reading the field

(in the future, we will switch to dispatch on the fieldtype rather than the current dictionary approach, but that is out of the scope of this commit.)