set GTSAM_SLOW_BUT_CORRECT_EXPMAP to ON by default and print deprecation warning

Author: varunagrawal

cmake/HandleGeneralOptions.cmake

@@ -47,7 +47,9 @@ option(GTSAM_ALLOW_DEPRECATED_SINCE_V43     "Allow use of methods/functions depr
 option(GTSAM_SUPPORT_NESTED_DISSECTION      "Support Metis-based nested dissection" ON)
 option(GTSAM_TANGENT_PREINTEGRATION         "Use new ImuFactor with integration on tangent space" ON)
 option(GTSAM_SLOW_BUT_CORRECT_BETWEENFACTOR "Use the slower but correct version of BetweenFactor" OFF)
-option(GTSAM_SLOW_BUT_CORRECT_EXPMAP        "Use slower but correct expmap for Pose2"  OFF)
+option(GTSAM_SLOW_BUT_CORRECT_EXPMAP        "Use slower but correct expmap for Pose2"  ON)
+
+message(WARNING "GTSAM_SLOW_BUT_CORRECT_EXPMAP has been deprecated and will be removed in a future release.")
 
 if (GTSAM_FORCE_SHARED_LIB AND GTSAM_FORCE_STATIC_LIB)
     message(FATAL_ERROR "GTSAM_FORCE_SHARED_LIB and GTSAM_FORCE_STATIC_LIB are both true. Please, to unambiguously select the desired library type to use to build GTSAM, set one of GTSAM_FORCE_SHARED_LIB=ON, GTSAM_FORCE_STATIC_LIB=ON, or BUILD_SHARED_LIBS={ON/OFF}")