Make sure we behave properly with noninteger time

[richfitz]

This PR fixes the code generation error spotted in #126, where the ODE interface was using size_t where it should have used real_type sometimes. Not too bad I think

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (7a62bec) 100.00% compared to head (bc71a27) 100.00%.

:exclamation: Current head bc71a27 differs from pull request most recent head 582192a. Consider uploading reports for the commit 582192a to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #127 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 7 7 Lines 1150 1151 +1 ========================================= + Hits 1150 1151 +1 ``` | [Impacted Files](https://codecov.io/gh/mrc-ide/odin.dust/pull/127?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide) | Coverage Δ | | |---|---|---| | [R/generate\_dust.R](https://codecov.io/gh/mrc-ide/odin.dust/pull/127?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide#diff-Ui9nZW5lcmF0ZV9kdXN0LlI=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide)

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