Add serialization version flag to the Dataset element in the DMR part of a DAP4 Data Response.

[ndp-opendap]

Now that we have discovered and fixed an order of serialization bug we need to provide a mechanism for clients to identify the servers that are "patched".

Proposal:
Add a serialization attribute to the Dataset element on the DAP4 Data Response:

serialization="dap-4.0" 

like this:

<Dataset 
    xmlns="http://xml.opendap.org/ns/DAP/4.0#" 
    xml:base="http://test.opendap.org:8080/opendap/NSIDC/ATL01_20201221111151_04882305_002_01.h5.dmr" 
    dapVersion="4.0" 
    dmrVersion="1.0" 
    serialization="dap-4.0" 
    name="ATL01_20201221111151_04882305_002_01.h5">

Let's modify/discuss/refine this here, before we try to fix it.

@Mikejmnez , @jgallagher59701 , @dh-opendap , @hannahilea

(We tried to use the dmrVersion attribute but the semantics didn't work so we are going to try and fix it, again)

[jgallagher59701]

We should probably do this.