COMP: Update Eigen GIT_TAG to fix checkout error

Eigen git tag which was currently used (e81ac4502ebbfde43a1e1761b36853d3ead47c33)
can not be checked out anymore as of Sunday, January 29 2017 for unknown
reasons, causing all builds on the dashboard to fail:

http://my.cdash.org/index.php?project=iMSTK&date=2017-01-29

error:
```
fatal: reference is not a tree: e81ac4502ebbfde43a1e1761b36853d3ead47c33
```

CMake/External/External_Eigen.cmake

@@ -4,7 +4,7 @@
 include(imstkAddExternalProject)
 imstk_add_external_project( Eigen
   GIT_REPOSITORY ${git_protocol}://github.com/RLovelett/eigen.git
-  GIT_TAG e81ac4502ebbfde43a1e1761b36853d3ead47c33
+  GIT_TAG 3.2.10
   CONFIGURE_COMMAND ${SKIP_STEP_COMMAND}
   BUILD_COMMAND ${SKIP_STEP_COMMAND}
   INSTALL_COMMAND ${SKIP_STEP_COMMAND}