Make a decision regarding Missing Data Indicator

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 #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.

Start by reviewing the unit test that hard-codes the Missing Data Indicator and the discussion in Issue #34. Determine whether the profiler should expose an MDI or report zero call_count_ as an error; done means the project has a documented decision and the affected test reflects it.

Written by the indexing model from the issue text.