Allow SIGINT set to be skipped via env variable.

This is relevant when attempting to use Dask outside of pyGSTi,
where signals cannot be set in the workers.
Setting the PYGSTI_NO_CUSTOMLM_SIGINT env variable
now skips this behavior.

pygsti/optimize/customlm.py

@@ -10,6 +10,7 @@
 # http://www.apache.org/licenses/LICENSE-2.0 or in the LICENSE file in the root pyGSTi directory.
 #***************************************************************************************************
 
+import os as _os
 import signal as _signal
 import time as _time
 
@@ -25,7 +26,10 @@
 # from scipy.optimize import OptimizeResult as _optResult
 
 #Make sure SIGINT will generate a KeyboardInterrupt (even if we're launched in the background)
-_signal.signal(_signal.SIGINT, _signal.default_int_handler)
+#This may be problematic for multithreaded parallelism above pyGSTi, e.g. Dask,
+#so this can be turned off by setting the PYGSTI_NO_CUSTOMLM_SIGINT environment variable
+if 'PYGSTI_NO_CUSTOMLM_SIGINT' in _os.environ:
+    _signal.signal(_signal.SIGINT, _signal.default_int_handler)
 
 #constants
 _MACH_PRECISION = 1e-12