update XML Schema definition

[duffee]

Make the following changes to PhysicalConstants.xml, PhysicalConstants.xsd and XML_Schema.md

• [x] change element url to source with href="url"

[duffee]

from https://culturedperl.com/perl-5-xml-validation-with-dtd-and-xsd-ec2d90f7c434 and https://www.tutorialspoint.com/xsd/xsd_numeric.htm

• should include attributeFormDefault="unqualified"
• change value, uncertainty, maxValue, minValue to numerical types
  • xs:decimal supports up to 18 digits
  • double and float are listed in https://www.w3schools.com/xml/schema_dtypes_misc.asp
  • xs:nonNegativeInteger [0,1,2 ..]
  • can place restrictions on values (length, whiteSpace, fractions)
• xs:anyURI for urls
• xs:boolean for ?

Test validation and report errors cleanly https://culturedperl.com/perl-5-xml-validation-with-dtd-and-xsd-ec2d90f7c434

[duffee]

Added a definition for the symbolRepresentation element

[duffee]

• Can't easily add numerical type checking into values because they are a Complex Type. Should change to remove the system attribute.
• Look at XML_Schema.md for suggestions on changing the data source (e.g. CODATA 2014) element, reference_standard to something better.