Include local xsd schema to avoid internet resource unavailability

[eblondel]

Following CRAN notice, the xml.xsd will be set locally to avoid issues, and make schemas loading faster

Dear maintainer,

Please see the problems shown on
[<https://cran.r-project.org/web/checks/check_results_atom4R.html>](https://cran.r-project.org/web/checks/check_results_atom4R.html).

Please correct before 2022-09-01 to safely retain your package on CRAN.

It seems we need to remind you of the CRAN policy:

'Packages which use Internet resources should fail gracefully with an informative message
if the resource is not available or has changed (and not give a check warning nor error).'

This needs correction whether or not the resource recovers.

The CRAN Team