mediafile: get rid of FloatValueField
mediafile: get rid of FloatValueField

At the risk of increasing line noise, I've replaced the specialized
FloatValueField (for ReplayGain tag encoding) with two more generalized options
on StorageStyle (float_places and suffix). This allows us to avoid an
unnecessary encode/decode round-trip when encoding for Sound Check.

google-code-export/beets,google-code-export/beets,google-code-export/beets