Updated to use absolute imports for packages rather than relative ones.

gprMax/fields_update.pyx

@@ -19,7 +19,7 @@
 import numpy as np
 cimport numpy as np
 from cython.parallel import prange
-from .constants cimport floattype_t, complextype_t
+from gprMax.constants cimport floattype_t, complextype_t
 
 
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