Create only one cell style for all observation rows instead of creating one style per row to fix error when exporting excel files.

GCP-10125

Reviewer: Crisjaira Villarino
Added Mockito as dependency for unit tests

GCP-10125

Reviewer: Crisjaira Villarino
Add unit test and create test data for exporting a study with more than 4000 observations

GCP-10125

Reviewer: Crisjaira Villarino
Removed commented codes, added curly braces to if statements, changed printstacktrace to log.error

GCP-10125

Reviewer: Crisjaira Villarino