Add test to check platform precision using a string + fix error (#551)

* add test to check platform precision using a string

* Remove import in function that was causing an unbound local var

openmmtools/tests/test_utils.py

@@ -45,6 +45,11 @@ def test_platform_supports_precision():
             if supported_precisions != set(['mixed']):
                 raise Exception(f"'CPU' platform should support 'mixed' precision, but platform_supports_precision reports {supported_precisions}")
 
+
+def test_string_platform_supports_precision():
+    """Test that if we use a string for the platform name, it works"""
+    assert platform_supports_precision("CPU", "mixed")
+
 # =============================================================================
 # TEST STRING MATHEMATICAL EXPRESSION PARSING UTILITIES
 # =============================================================================

openmmtools/utils.py

@@ -544,10 +544,6 @@ def platform_supports_precision(platform, precision):
         return precision in ['mixed']
 
     if platform.getName() in ['CUDA', 'OpenCL']:
-        try:
-            import openmm
-        except ImportError:  # OpenMM < 7.6
-            from simtk import openmm
         properties = { 'Precision' : precision }
         system = openmm.System()
         system.addParticle(1.0) # Cannot create Context on a system with no particles