add back obs_weights to est_tml()

[PhilBoileau]

Addresses #44

[PhilBoileau]

Hey @nhejazi! Checks are passing locally. The check failures seem to be due to a missing package:

ERROR: package installation failed
Error: Error: Failed to install 'sl3' from GitHub:
  ! System command 'R' failed
In addition: Warning message:
package ‘imputeMissings’ is not available for this version of R

Should we try to address this issue right now, are is the sl3 already aware of it?

[PhilBoileau]

These failures seem to be due to issues with installing sl3 from GitHub, @nhejazi. All checks pass locally, however.