Evaluate assert/discriminator expressions after groupContent: WIP

[jadams-tresys]

According to 9.5 of the DFDL spec assert and discriminators with expressions should be processed after the content of their enclosing sequence, group, or choice. Before these expressions were always being processed before the content.

This commit also moves the setVariable expression evaluaiton to the correct place, which is before the enclosing group.

DAFFODIL-1971, DAFFODIL-1590

[jadams-tresys]

This is primarily WIP as it needs more test coverage and I have been informed that I need to hold off on working on this for a little while.

It is fully functional, just needs more tests for edge cases surrounding evaluating expressions after the content of sequences, groups, and choices.

Also need to add tests for the setVariable change.