Provide support for vendor specific extensions

[PeterCDMcLean]

This is a feature request for adding support to parse major vendor extensions (Xilinx) to ipyxact.

Below is an example of a fileSet extension that specifies a dependency between a Xilinx core to a 'subCore'

    <spirit:fileSet>
      <spirit:name>xilinx_utilityxitfiles_xilinx_com_ip_xbip_utils_3_0__ref_view_fileset</spirit:name>
      <spirit:vendorExtensions>
        <xilinx:subCoreRef>
          <xilinx:componentRef xilinx:vendor="xilinx.com" xilinx:library="ip" xilinx:name="xbip_utils" xilinx:version="3.0" xilinx:coreRevision="10">
            <xilinx:mode xilinx:name="read_mode"/>
          </xilinx:componentRef>
        </xilinx:subCoreRef>
      </spirit:vendorExtensions>
    </spirit:fileSet>

[olofk]

I think ipyxact should be able to at least parse vendorextensions. Then it would be up to the actual application to do something sensible with it. E.g. FuseSoC could perhaps make use of this information for dependency managment. I think the first step is to ensure that the new API can handle vendorextensions properly