boss: allow resolutions that are based on a floating point minimum

[jstriebel]

Currently wk-connect assumes that all resolutions are integers, which is not always the case for Boss datasets. This PR adds a Vec3Df datatype and uses it for the global scale and the Boss resolution normalization. I tested this locally.

Possible dataset to test this (min resolution 3.8, 3.8, 50.0):

{
    "boss": {
        "Connectomics_Department": {
            "acardona_0111_8": {
                "domain": "https://api.boss.neurodata.io",
                "collection": "cardona",
                "experiment": "acardona_0111_8__12_03_2017",
                "username": "…",
                "password": "…"
            }
        }
    }
}