experiment.A normal user should never need to call this function.
init_experiment(experiment, alignment_params)an `experiment`
a pirouette experiment, as can be created by create_experiment
parameters to simulate an alignment, as can be created by create_alignment_params
It does the following:
if an MCMC's treelog filename is $(tree).trees,
replace it to by a full path using complete_treelog_filename
if an MCMC's tracelog filename is NA,
replace it to [alignment_folder]/[alignment_id].log
Both is done for the regular MCMC in experiment$inference_model and in
experiment$est_evidence_mcmc.
[alignment_id] is obtained by using
get_alignment_id on the alignment_params$fasta_filename.
[alignment_folder] is obtained by using dirname
on alignment_params$fasta_filename