Fill Value Dense Arrays

[lukecampbell]

For parameters that are missing observations, can we fill them completely with fill values?

So, if the parameters are explicitly or implicitly requested they always return an array of the same size as the time values.

[caseybryant]

coverage_model commit 46aa2a5a6896d88e3c43e88f0d2a953a469a877e submitted to resolve this issue.