Enable calculation of unnormalized LBI

[huddlej]

Historically, we have calculated LBI and then normalized all LBI values by the
maximum value in a tree. However, there are cases when users would prefer to
analyze the raw LBI values calculated prior to normalization. For this reason,
this commit modifies the existing LBI function and command line interfaces to
accept an argument that will disable normalization. For backwards compatibility,
normalized LBI output remains the default.

One side effect of this commit is to remove unused kwargs from both the
calculate_LBI and select_nodes_in_season functions.

[jameshadfield]

Cool -- quick heads up that the colour scale bounds for lbi of 0 & 0.7 are hardcoded into auspice here https://github.com/nextstrain/auspice/blob/v2/src/util/colorScale.js#L197 so the un-normalised LBI may be confusing if it's still called lbi 😉

One day it'd be great to define desired bounds of continuous variables in the auspice JSONs, but we're not there yet.

[huddlej]

Thanks @jameshadfield ! I just made an issue for this in auspice, so we don't forget. In the meantime, users who want to export unnormalized LBI for visualization in auspice can specify a different attribute name with the required --attribute-names flag.

Since this PR doesn't change the defaults and the special treatment of the lbi attribute is in auspice, I'm ok with not adding any additional documentation here about the limitations of the lbi attribute.

[rneher]

this looks good to me.