Simplify configuration of grid property keywords like PORO and ACTNUM

Grid properties like PORO all have the characteristic:

1. Fixed size - one record.
2. One item only - which has sizeType == ALL

In principle this could be configured with the json:

    {"name" : "PORO" , "size" : 1 , "items" : [{"name" : "????" , "value_type" : "FLOAT" , "size_type" : "ALL"}]}

With the following PR this can now be configured as:

   {"name" : "PORO" , "data" : {"value_type" : "FLOAT"}} 

Optionally a "default" : can be included in the "data" : section. In addition to the simpler configuration this PR will set a isDataKeyword property on the Keyword. Have also added methods to get vector data from a deck item.