allow users to estimate the ITRs with their own models

[xiaolong-y]

No problem setting quantities of interest directly from run function now but quantities are only returned as a list of unnamed elements.

[xiaolong-y]

Naming the elements in the output

• in output$qoi: each outcome list has outcome variable name

[jialul]

• [x] Next step: allow users to estimate the ITRs using their own models + their own evaluation metrics

[jialul]

Incorporated caret package functions, which allow users to design their own models

[jialul]

Finished this task in commit 4cd36a7