Added mean function

[cpfiffer]

Allows you to do stuff like

# Returns a dataframe with just parameters and their means.
mean(chn)

# Returns either a dataframe (if more than one parameter)
# or a single value.
mean(chn, :x)

# Returns a dataframe.
mean(chn, [:x, :y])

The above also works with Strings. I've also made a redundant Chains constructor a little more helpful by converting a 2d array to a 3d one, so you don't have to make an AxisArray or a 3d array yourself if you're passing in custom values.