perf : disable ndarray by default

[usaxena-4md]

Summary

Flips the default value of the GS_ENABLE_NDARRAY environment variable
from "1" to "0", so ndarray is disabled by default unless a user
explicitly opts in by setting GS_ENABLE_NDARRAY=1.

Change

In genesis/__init__.py, init():

# before
is_ndarray_disabled = os.environ.get("GS_ENABLE_NDARRAY", "1") == "0"
# after
is_ndarray_disabled = os.environ.get("GS_ENABLE_NDARRAY", "0") == "0"

[peizhang56]

If we disable GS_ENABLE_NDARRAY, the unit test will run much slower.
I would not change it to 0, instead, user can use performance mode to set it 0 by env