* base.R (write_keyval_file): New function.

* resources.R (METFRAG_ADDUCT_SWITCHES): Fix a typo.

* state.R (new_runtime_state): Update results with the new field
`subpaths'. Also, opted for relative vs absolute paths, because the
latter do not play well with tests in (stochastic) temporary
directories.

(base_conf): Update result to have a class `conf' signature.

(metfrag_conf): Update with new parameters.

(new_conf): Update with a call to `metfrag_conf'.

(write_metfrag_config): New function which writes the MetFrag config
file.

* tests/testthat/test-metfrag.R: New file with MetFrag tests.

* tests/testthat/_snaps/metfrag.md: New file .