Make a decision regarding Missing Data Indicator

[bencopl]

I currently have a unit test that checks for an MDI at certain time points where the MDI is hard-coded into my test. It's probably a temporary implementation as I don't think we've made a formal decision about having an MDI. Things to consider:

• Do we even want an MDI, and if so, what value should it have and should it be coded as a const value within the profiler?
• An alternative which links in nicely with Issue Add number of calls to profiler output. #34 is to simply throw an error if call_count_ is zero, or something similar. In which case I can get rid of the MDI stuff in my unit test.