Added clear() to DecimalField in Open XAL. Replaced setNull() from XAL.
Added clear() to DecimalField in Open XAL. Replaced setNull() from XAL.

EuropeanSpallationSource/openxal,openxal/openxal,EuropeanSpallationSource/openxal,EuropeanSpallationSource/openxal,openxal/openxal,openxal/openxal,openxal/openxal,openxal/openxal,luxiaohan/openxal-csns-luxh,EuropeanSpallationSource/openxal,EuropeanSpallationSource/openxal,luxiaohan/openxal-csns-luxh,luxiaohan/openxal-csns-luxh,luxiaohan/openxal-csns-luxh