Removed IsoSampleToChunkBox.remove_entries_before_chunk()

Was replaced by remove_chunk_refs_before().
Implementation of IsoSampleSizeBox.remove_sample_refs_before()

IsoSampleSizeBox.remove_samples_before() simply shifts down the elements in
 the offset array such that the given sample reference is the first sample
 in the list.

Tested sample removal with all tracks on non-segmented DLNA reference
 streams and verified SampleSizeBox format by examination.
Further refinement of IsoSampleTableBox.remove_sample_refs_before_point()

Added support for handling calculation of samples that are mid-chunk.

Added IsoTimeToSampleBox.remove_sample_refs_before() and
 IsoSampleSizeBox.count_samples_for_bytes() to support
 remove_sample_refs_before_point().

All throws of IsoBoxError.ENTRY_NOT_FOUND now include the class/function
 name.

Revised IsoChunkOffsetBox.chunk_for_offset() to return the chunk's
 byte offset (so the intra-chunk offset can be calculated by the caller)

Did forward and backward tests with random access samples to ensure
 the byte offset calculation for the sample, passed into the
 remove_sample_refs_before_point() logic, produces the same sample
 number. Also tested with offsets that don't land in chunks (which
 should be the normal case when sample numbers aren't provided)