Enable calculation of unnormalized LBI
Enable calculation of unnormalized LBI

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.

nextstrain/augur,blab/nextstrain-augur,nextstrain/augur,nextstrain/augur