Update highlevel.py

Converted assert's into warnings. Reason:

in some cases, storing an edf file fails because of roundoff errors:
e.g. `phys_min -3565.82 vs. signal_min 3565.8200000000000006` will fail in write_edf (but shouldn't)

pyedflib/highlevel.py

@@ -531,12 +531,12 @@ def write_edf(
             assert pmin != pmax, \
             f'physical_min {pmin} should be different from physical_max {pmax}'
         else: # only warning, as this will not lead to clipping
-            assert pmin<=sig.min(), \
-            'phys_min is {}, but signal_min is {} ' \
-            'for channel {}'.format(pmin, sig.min(), label)
-            assert pmax>=sig.max(), \
-            'phys_max is {}, but signal_max is {} ' \
-            'for channel {}'.format(pmax, sig.max(), label)
+            if pmin > sig.min():
+                warnings.warn(f'phys_min is {pmin}, but signal_min is {sig.min()} ' \
+                'for channel {label}')
+            if pmax < sig.max():
+                warnings.warn(f'phys_max is {pmax}, but signal_max is {sig.max()} ' \
+                'for channel {label}')
 
     # get annotations, in format [[timepoint, duration, description], [...]]
     annotations = header.get('annotations', [])