tiny detail in delta attrs

[juliettelavoie]

https://github.com/Ouranosinc/xscen/blob/2f8d143997b00f935590e792b133a0aedce0ce8b/xscen/aggregate.py#L263

This is really a small issue, but in the description and longname of delta, I would prefer a "-", not a "".

[RondeauG]

Sure. I honestly don't remember why we did that. Maybe to separate the periods more clearly?

[juliettelavoie]

I think it might have just been copied from the variable name: var_delta_YYYY_YYYY. We can't have "-" in variable name, but that's not the case for attrs.

[RondeauG]

Fixed by #228.