Clean copies of GridMath and FieldImpl

from latest IDV/VisAD 2014-05-14

[1720]
Initial proof of concept: keep metadata in FI

- FieldImpl.java:  add metadata HashMap field

- background.py: scripting hooks to utilize new metadata field
    - Get rid of binary overrides... doing e.g. "sub(a,b)" will now just
      return a plain FlatField, but layer labels will still work due to
      retained metadata.  So the binary overrides should be unnecessary.
    - createLayer gets layer labels from new FieldImpl metadata,
      not _MappedData dictionary
    - _MappedData constructor initializes FieldImpl metadata

- GridMath.java: demo of keeping track of metadata in "subtract"

[1720]
Merge remote-tracking branch 'upstream/master' into metadata
Implement Jon's suggestions

- Move metadataMap initialization into constructor
- use ConcurrentHashMap
- fix metadataMap setter (nice catch Jon!)

[1720]
Merge pull request #90 from mhiley/metadata

Initial proof of concept: keep metadata in FieldImpl