only set wrapper version when htsjdk is root project

[ssadedin]

Description

Currently, htsjdk cannot be built as a submodule of a parent project that also uses gradle. This happens because Gradle only supports the use of the Gradle "wrapper" in a root project, and not in child projects.

See: https://github.com/gradle/gradle/issues/5816

This change puts conditional logic around the wrapper task configuration in the build.gradle file so that it is only invoked when htsjdk is built as a root project (this is applying the workaround suggested in the issue above).

Things to think about before submitting:

• [X ] Make sure your changes compile and new tests pass locally.
• [ X] Add new tests or update existing ones:
  • A bug fix should include a test that previously would have failed and passes now.
  • New features should come with new tests that exercise and validate the new functionality.
• [ X] Extended the README / documentation, if necessary
• [ X] Check your code style.
• [ X] Write a clear commit title and message
  • The commit message should describe what changed and is targeted at htsjdk developers
  • Breaking changes should be mentioned in the commit message.

[codecov-commenter]

Codecov Report

Merging #1492 into master will not change coverage. The diff coverage is n/a.

@@             Coverage Diff             @@
##              master     #1492   +/-   ##
===========================================
  Coverage     69.279%   69.279%           
  Complexity      8764      8764           
===========================================
  Files            590       590           
  Lines          34755     34755           
  Branches        5800      5800           
===========================================
  Hits           24078     24078           
  Misses          8386      8386           
  Partials        2291      2291