Fix lower limit checking of FloatRange

compatible used sys.float_info.min (smallest representable positive
float) instead of -sys.float_info.max.
so FloatRanges -500,10 and -10,10 were two-way compatible
Since it is correctly set in __init__, no need for the guard here

Change-Id: If693fa69a8b2de1aa52ce702bd282a84a8f4c55a
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/31056
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Georg Brandl <g.brandl@fz-juelich.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>

test/test_datatypes.py

@@ -658,6 +658,7 @@ def test_get_datatype():
     (FloatRange(-10, 10), FloatRange()),
     (IntRange(-10, 10), FloatRange()),
     (IntRange(-10, 10), IntRange(-20, 10)),
+    (FloatRange(-10, 10), FloatRange(-15, 10)),
     (StringType(), StringType(isUTF8=True)),
     (StringType(10, 10), StringType()),
     (ArrayOf(StringType(), 3, 5), ArrayOf(StringType(), 3, 6)),