Config override bug

[jagzx]

Bug location: https://github.com/alpha-xone/xbbg/blob/main/xbbg/io/param.py#L52
Fix: Drop any duplicates and keep last (since our own config files are listed last in config_files). I would want my override to take priority over default.

config = (
    pd.concat([
        load_yaml(cf).apply(pd.Series)
        for cf in cfg_files
    ], sort=False).drop_duplicates(keep="last")
)

Perhaps the same should be done for assets and ccy.