Extracts the bundled testdb tarball under dir and points the
active groot at <dir>/trackdb/test. Used by examples and tests.
gdb.init_examples(dir = NULL)Directory under which to extract the example trackdb. Created if
it does not exist. Defaults to Sys.getenv("MISHA_EXAMPLES_DIR") if
set, otherwise tempdir().
None.
If dir is NULL (default), uses the value of environment
variable MISHA_EXAMPLES_DIR, falling back to tempdir().
Set MISHA_EXAMPLES_DIR to a roomy path when /tmp is tight.