Add the Wtime function to the MPI wrapper (to avoid #ifdefs)

[pcarruscag]

Proposed Changes

Add the Wtime function to the MPI wrapper (to avoid #ifdefs)

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags).
• My contribution is commented and consistent with SU2 style.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp) , if necessary.

[economon]

Good idea and straightforward. LGTM