Pass the tree rule's mem_mb through to IQ-TREE

While most of our mem_mb definitions are only heuristics for Snakemake's
scheduler and the commands themselves aren't limited or aware of the
mem_mb defined, IQ-TREE *does* support memory limits.

Resolves <#949>.

workflow/snakemake_rules/main_workflow.smk

@@ -811,7 +811,7 @@ rule tree:
         """
         augur tree \
             --alignment {input.alignment} \
-            --tree-builder-args {params.args} \
+            --tree-builder-args {params.args}' --mem {resources.mem_mb}M' \
             {params.exclude_sites} \
             --output {output.tree} \
             --nthreads {threads} 2>&1 | tee {log}