pressure functionObject, pName is not read

Hello,

The pressure functionObject does not read p entry which represents pName, although it is mentioned in the description of the class. The pressure field name is hardcoded and passed to fieldExpression here. In order to have the right behaviour, the user can set the field to pName, then the read function of fieldExpression will overwrite the field name. I guess the easiest fix here is to update the description of the class and replace p with field, but it isn't the most intuitive.