libcbcf classes should implement Python3-style dict methods rather than Python2-style

[jmarshall]

Several classes in libcbcf.py, namely

VariantHeaderRecord      VariantRecordFilter     BaseIndex
VariantHeaderMetadata    VariantRecordFormat
VariantHeaderContigs     VariantRecordInfo
                         VariantRecordSamples
                         VariantRecordSample

implement the following dict methods:

• iterkeys() / itervalues() / iteritems() returning iterators; these are implemented using libhts primitives
• keys() / values() / items() returning lists; implemented in terms of iterkeys() et al

These are Python2-style dictionary methods.

They should be reworked to provide a Python3-style interface:

• keys() / values() / items() returning some new view objects, implemented using libhts primitives
• iterkeys() / itervalues() / iteritems(), deprecated, implemented in terms of keys() et al