Revert namespace change in XSD

[EzraBrooks]

Please merge #227 first so the diff on this PR is less wonky :) I'll make this not a draft and rebase it once that's merged.

Reverts a change from #200 that, while well-intentioned, appears to me to make it harder to validate the XSD with traditional tools. The change made the recommended way of using the XSD the <? xml-model ?> processing instruction, which is non-standard and not broadly supported.

By way of evidence, using the popular xmlschema library in Python:

PS C:\Users\ezram\urdfdom\xsd> gc -Head 5 .\autogenerated.urdf
<?xml version="1.0" encoding="utf-8"?>
<?xml-model href="https://raw.githubusercontent.com/aminya/urdfdom/xsd/xsd/urdf.xsd"?>
<robot name="name1" version="1.0" xmlns="http://www.ros.org">
 <joint name="name1" type="type1">
   <mimic joint="joint1" multiplier="1" offset="0" />

PS C:\Users\ezram\urdfdom\xsd> pipx run --spec xmlschema  xmlschema-validate .\autogenerated.urdf
cannot get a schema for XML data, provide a schema argument

PS C:\Users\ezram\urdfdom\xsd> gc -Head 5 .\autogenerated.urdf
<?xml version="1.0" encoding="utf-8"?>
<robot name="name1" version="1.0" xmlns="http://www.ros.org" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.ros.org urdf.xsd">
  <joint name="name1" type="type1">
    <mimic joint="joint1" multiplier="1" offset="0" />

PS C:\Users\ezram\urdfdom\xsd> pipx run --spec xmlschema  xmlschema-validate .\autogenerated.urdf
.\autogenerated.urdf is not valid

I also checked the source code of lxml and found no reference to xml-model.

I understand that the upside of this change, if you're using a schema validator that accepts the xml-model processing instruction, is that you can validate the robot element itself - whereas with the original implementation the robot element is the element referencing the schema location, so it isn't going to be validated. However, I think remaining compatible with the greater ecosystem of tools is more important than the convenience factor here.