Merge SDMXMessageFooter.xsd and SDMXMessage.xsd

[Tzaphkiel]

Received from IMF

Currently the header specifications are stored in SDMXMessage along with message specifications while footer specifications are split into a separate file.

We’d like to treat headers and footers similarly, simplify the XSD structures, as well as the SDMX message structure.

Because of this split today many SDMX files need to declare an additional namespace just for footers.

[stratosn]

Need to review.