DPath: Query-style expressions - was: fn:count cannot be used to count items in a sequence

Note: This is an issue with the dpath implementation

This is similar to DFDL-1075, but different enough to be its own ticket.

Previously, you could use the fn:count function to count the items in a sequence. e.g if you had a sequence like the following:
<types_of_fruits>
<apples>5</apples>
<oranges>4</oranges>
<pears>1</pears>
</types_of_fruits>
You could specify fn:count(types_of_fruits) and it would return 3. If you try that with the DPath implementation, you get the following:

[error] Test edu.illinois.ncsa.daffodil.section23.dfdl_expressions.TestDFDLExpressions.test_more_count_0 failed: java.lang.Exception: Schema Definition Error: Subset The count function must reference a single array.

See tests:
test_more_count_0
test_more_count_1
test_more_count_2
test_more_count_1b_2
test_more_count_1b

(these are in the serialization-with-dpath4-testcleanup branch, and have not yet been pulled into 0.15.0)