adding in segment specific minimum lengths for filtering

[miparedes]

This is response to issue #4 which requires specific minimum lengths for each separate oropouche segment.

This is done by adding in separate filtering steps for each segment and having a separate filter_{segment} flag in the config file.

I'm going to tag @joverlee521 here because I'm not sure if there's a standardized way of doing this that's not my kinda hacky solution

These new trees are up on staging: https://nextstrain.org/staging/oropouche/L https://nextstrain.org/staging/oropouche/M https://nextstrain.org/staging/oropouche/S