Parse the XML files via xmlschema and the XSD definitions

[alexamici]

Most metadata are distributed in XML files (e.g. manifest.safe, product/swath and calibration files) and the product zip file contains part of the XSD definition to use.

As the XSD schemas are required when using xmlschema we need a way to always have a set of files. Strategy is:

1. allow the user to specify a set of XSD files as a mapping from namespace to path to the schema file
2. look for the distributed schema files based on a predefined product folder structure e.g. top_dir/support/s1-level-1-product.xsd
3. use a mapping from namespace to shipped schema file

[alexamici]

3. is mostly closed by Start using xmlschema for parsing #6. Also Add an API to access as much original metadata as possible #7 is related.

[aurghs]

I think #11 is the last small step for closing this issue.