Fixed Nesting and Added in a few more jmeter elements

Whilst working with the DSL recently we noticed that in some cases the nesting of elements did not come out correctly in Jmeter

In this pull request we have added a few basic tests to show this and also added the fix

In addition we have added the support of the following elements:
* random_variable
* aggregate_report
* Switch controller
* random_order controller
* simple controller
* interleave controller