GenCode.php and XML Schema Files - Schema Versioning

In order to maintain a history of schema modifications - and facilitate creation of schema update scripts - we need to implement and support schema versioning tags (<add>, <drop>, <alter>) in the xml schema files and in GenCode.php.

Individual.xml has been updated as a sample for the schema versioning tags. Fields which existed in 1.1 and have been dropped for 1.2 (e.g. gender - which was replaced by gender_id) have been tagged as follows:
<add>1.1</add>
<drop>1.2</drop>

The following tasks need to be done:
1. Update the other modified/new xml schema files with the appropriate tags (for example - Import_Export_Mapping tables need <add>1.2</add> tags, Gender.xml <add>1.1</add> tags are incorrect (s/b 1.2), etc.

2. Modify GenCode.php to require a "build version" parameter and use the version number to determine which schema tables and columns are built. Tables/columns with <add> value > = build version are ncluded.Tables/columns with <drop> value < = build version are NOT included. (Drop over-rides Add).