Minors fixes and enhancements

1. Updated to enhance value parsing for expected value and input value.
2. Fixed value conversion and enhanced validation for >, >=, <, <= operator.
3. Updated expectations for null value for <, <= operators.
4. Fixed the parameter validator for decimal-list.
5. Added method to find child from XML document at a particular index.

Upgrade Old STAS project to new version 6.0.7:

1. Open pom.xml file from your STAS project and change the following elements' value to 6.0.7:
   <version>6.0.7</version>
<smart-testauto-framework.version>6.0.7</smart-testauto-framework.version>
<smart-testauto-cucumber-stepdefs-en.version>6.0.7</smart-testauto-cucumber-stepdefs-en.version>

2. You are all done.