BF: Print error message from CLI if no dicom files found in a dir

Fixes Issue #9
TST: Added test that demonstrates the bug
BF+TST: Fix issue merging meta w/ per slice and meta w/o per slice.

Expanded test case for the fix.
Release version 0.6.2
ENH: Handle series with orthogonal slices (i.e. localizers)
ENH: Reduce peak memory usage for CLI when converting mutiple directories.
ENH: Reduce memory usage and run time when not embedding/dumping meta.

Added new extractor that only extracts the minimal set of meta data
needed for conversion to Nifti.
gitignore