Hide FieldDescriptor (a DBase3 internal format structure)

The org.apache.sis.storage.shapefile package contains a FieldDescriptor public class, which contain information that look likes very specific to the internal of DBase format. For example:

• Field name as an array of bytes (this is not what user would usually handle, which are rather String objects)
• Field address in memory (maybe an heritage from C/C++ ?)
• DBase+ Lan Reserved 2 (not sure what it is, documentation only said "reserved 2")

Those internal details should not be visible to the users. I suggest to either declare the class package-privated, or move it to an internal package.

The FieldDescriptors class, which is a list of FieldDescriptor, should also move in the same way.