Parsing of grouped elements

[nilskruthoff]

Currently the parsing is surficial and only children of <p:spTree> nodes are parsed. This excludes every node that is grouped and nested under <p:grpSp> nodes.
we change the parser to recursively parse nodes under <p:grpSp>:

<p:grpSp>
    <p:sp>...</p:sp>            // now recognized
    <p:grpSp>
        <p:pic>...</p:pic>      // now recognized
        // etc..
    </p:grpSp>
</p:grpSp>

• parse_group(node) processes a single element.
• If the element is a known element type (e.g. sp, graphicFrame, pic), it is parsed as normal.
• If the element is a group (grpSp), the function recursively calls all child elements of the group.
• Returns a complete, flat-extracted hierarchy of all contained elements regardless of the group depth in the XML.