Reduce cache size

[lhjohn]

Resolves issue https://github.com/OHDSI/DeepPatientLevelPrediction/issues/88.

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (ff9e22e) 99.59% compared to head (ad5b413) 99.59%.

:exclamation: Current head ad5b413 differs from pull request most recent head 53af472. Consider uploading reports for the commit 53af472 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #90 +/- ## ======================================== Coverage 99.59% 99.59% ======================================== Files 9 9 Lines 488 493 +5 ======================================== + Hits 486 491 +5 Misses 2 2 ``` | [Files](https://app.codecov.io/gh/OHDSI/DeepPatientLevelPrediction/pull/90?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OHDSI) | Coverage Δ | | |---|---|---| | [R/Estimator.R](https://app.codecov.io/gh/OHDSI/DeepPatientLevelPrediction/pull/90?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OHDSI#diff-Ui9Fc3RpbWF0b3IuUg==) | `100.00% <100.00%> (ø)` | |

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[egillax]

Could you add a unit test for this?

[lhjohn]

Resolves issue https://github.com/OHDSI/DeepPatientLevelPrediction/issues/88.