Non-generic IDatumConverter & IDatumConverterFactory

[mfenniak]

Add non-generic interfaces for IDatumConverter and IDatumConverterFactory. This is likely a pre-req for removing reflection in some other issues (#69, #67, #66) and fixing the dynamic invocation in issue #133.

public interface IDatumConverter
{
    object ConvertDatum(Datum datum);
    Datum ConvertObject(object @object);
}

public interface IDatumConverter<T> : IDatumConverter
{
    new T ConvertDatum(Datum datum);
    Datum ConvertObject(T @object);
}

public interface IDatumConverterFactory
{
    bool TryGet(Type datumType, IDatumConverterFactory rootDatumConverterFactory, out IDatumConverter datumConverter);
    bool TryGet<T>(IDatumConverterFactory rootDatumConverterFactory, out IDatumConverter<T> datumConverter);
}