Megahit presets option

megahit command inside the assembly module is written with the parameter --presets meta-sensitive.

While it offers sensitivity to assembly, neither large co-assemblies nor too complex metagenomics samples are well-suited to use this preset option, as the computation time and the complexity may scale quickly. In these cases, --presets meta-sensitive should be the preferred option instead.

As the config file was written in such a complete form to allow full parameterization, megahit presets option should be leave to the user's choice.