Unify concrete and references updates

[raulmonti]

Currently, concrete updates and reference updates have their own slots both in pspec and bpmn4s. It often happens that a data structure (datatype) has both concrete and reference fields together. This brings unfortunate situations as the following example:

In the concrete update we need to write:

some_data_store := SomeDataType {
    some_field = <SomeSubDataType[]>[ 
        SomeSubDataType{value = some_input_data_store.some_other_field},
    ]
}

In the references update we need to write:

x1: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_step_ouput.some_step_ouput_field['id1'],0)})
x2: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_step_ouput.some_step_ouput_field['id2'],0)})
x3: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_step_ouput.some_step_ouput_field['id3'],0)})
x4: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_step_ouput.some_step_ouput_field['id4'],1)})
x5: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_step_ouput.some_step_ouput_field['id5'],2)})
x6: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_step_ouput.some_step_ouput_field['id6'],3)})
x7: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_other_step_ouput.some_other_step_ouput_field, 0)})
x8: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_other_step_ouput.some_other_step_ouput_field, 1)})
x9: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_other_step_ouput.some_other_step_ouput_field, 2)})
x10: some_date_store.some_field := add(some_date_store.some_field, SomeSubDataType{value = get(some_other_step_ouput.some_other_step_ouput_field, 3)})

It would be preferable to just define a single update for some_data_store, and just deduce in the back which of the values need to be referenced and which are concrete. Then we could write:

A single update slot:

some_data_store := SomeDataType {
    some_field = <SomeSubDataType[]>[ 
        SomeSubDataType{value = some_input_data_store.some_other_field},
        SomeSubDataType{value = get(some_step_ouput.some_step_ouput_field['id1'],0)},
        SomeSubDataType{value = get(some_step_ouput.some_step_ouput_field['id2'],0)},
        SomeSubDataType{value = get(some_step_ouput.some_step_ouput_field['id3'],0)},
        ...
    ]
}