Read NMEXT and NMXML relative to project directory

[kylebaron]

Summary

As a user, I want NMXML and NMEXT to find nonmem results relative to the project directory, not the working directory.

Tests

• tests/testthat/test-nmxml.R
  • read nm estimates relative to cpp file

[kylebaron]

Ideally this would be the default behavior; but it's not. We'd like to eventually be at the point where both calculate the path relative to project but will take some time.