Modified preseq plot_complexity_curves.py script

**Note** - this is just to make it clear what has changed in this script. After it's been merged I'll port the script across into the [visualisations](https://github.com/SciLifeLab/visualizations) repo...

Updates:
- Rewrote how the argparsing worked
 - Just supplying filenames without having to use a `--ccurves` parameter
 - Making output filename have a default and not be required
 - Changing how the variables are delivered to `main()`, now in a named way
- Legend updates
 - Legend now below plot
 - File extensions truncated from sample names
 - If sample name starts with `accepted_hits_sorted_dupRemoved_`, this is truncated
 - Legend columns, margins and plot size are dynamically determined by size of legend text
- Axes updates
 - Font size increased
 - Axes ticks now on the outside
 - Labels changed
I would use Unique Molecules