Introduce a common syntax for parallel operations

I propose adding the following syntax extensions.

ds' := UNORDERED(ds) - implies the that order of ds is not required

The following attributes available on all activities:

• PARALLEL[(n)] execute this activity in parallel. The exact meaning may depend on the activity. Optional number of strands.
• ORDERED(bool) - is the order of any output relied on.
• UNORDERED - the order of output rows for all outputs cannot be relied on. (Synonym for ORDERED(FALSE))

The following are relevant on a subset of activities:

• STABLE(bool) - indicates if the order of the input rows are significant.
• UNSTABLE - equivalent to STABLE(false)
• ALGORITHM(x) - allow the algorithm to be set independently of the STABLE flag

Note: It is more common to know that the order of the input dataset is not required at the point of use (and fits in better with reusing definitions) - which is why UNORDERED(ds) is useful.

Derived order (e.g., from HPCC-10144) is annotated by adding an unordered attribute to an activity.

Other related changes:
SORT, UNSTABLE implies that the input dataset is unordered.

(+)(a,b[,options]) as a functional form of the append operator.
filter(ds, c1, c2, options) as a functional form of a filter.

Great care will be needed to ensure that the options aren't lost of activities when they are optimized - especially IF and filter.