Allow for merging of SCEs by integration group

[cbethell]

Issue Addressed Closes #340

What is the purpose of these changes?

The purpose of this PR is to allow users to specify which integration group each SCE belongs to (e.g. libraries from the same disease are merged together rather than all libraries being merged together into one).

What changes did you make?

The following modifications were made to the merge-sce.R script in the data integration module:

• an argument to take the desired integration_group
• a step to filter in the input metadata on that integration group argument

These modifications allow users to indicate which libraries (SCEs) should be grouped together.

The example metadata file was also updated to included an integration_group column and filled in with generic values.

Any comments, concerns, or questions important for reviewers Does this PR see to adequately address #340?

Checklist

Place an x in all boxes that you have completed.

• [x] I have run the most recent version of the code
• [ ] If I am adding in reports or generating any plots, I have reviewed the necessary reports
• [ ] I have added all necessary documentation (if applicable)

[cbethell]

Thanks for the review @allyhawkins! I added in that extra check and removed the additional ids from the example metadata as suggested -- let me know what you think now.