Easy getter methods for the file meta table

[Enet4]

The attributes in the struct dicom_object::meta::FileMetaTable retain the padding from the encoded DICOM file meta group, and so they are usually padded for even length, even when built via the builder. While this makes it easy to serialize, it is not very useful when inspecting the attributes.

This calls for new methods to FileMetaTable which automatically trim the padding character out before returning the string slice.

Example signature (should be one per textual attribute):

fn transfer_syntax(&self) -> &str { unimplemented!() }