Remove `default_mem_mb` from the cookie-cutter

[leoisl]

See #45 (comment) and #45 (comment)

[mbhall88]

and #41

[leoisl]

Sorry, I embarrassingly had not fully understood snakemake --profile parameter although I contribute to this project :( . After re-reading it and playing with some directives in the config.yaml, I realised we can actually specify not only default mem_mb, but also disk_mb and tmpdir. Closing this in favour of #50