Handle eta2

[cissard]

I've been facing an issue on my MA with F that have their first degree of freedom superior to 1 (e.g. main effects with more than 2 conditions). I have several papers that I couldn’t not include because of this. That’s really a shame since these papers often report the right amount of details. Up to now I have coded them in two separate columns, one for F with df1>1, and the other for its associated degrees of freedom (separated by comma) to keep the information somewhere. This issue was also pointed by a new contributor.
Maybe we could implement this with eta2? However this might require to convert all ES to eta2?

What do you think?