Add vector list datatype & add configurable SPACE_DIRECTIONS_TYPE option

[addisonElliott]

Changes

• Add int vector list and double vector list datatypes that are lists of Numpy arrays or None.
  • These new types are similar to their int matrix and double matrix counterparts except they are not 2D Numpy matrices.
• Add nrrd.SPACE_DIRECTIONS_TYPE to enable switching the datatype for the space directions field.
  • Valid options are double matrix or double vector list. The current default is double matrix for backwards compatibility but will be switched to double vector list in the next major release.
  • double vector list is superior over double matrix because it doesn't have the confusing row-of-NaN's representation and it doesn't imply an affine transform by being a matrix.
• Support row-of-None in addition to row-of-NaN for parse_optional_matrix & format_optional_matrix in addition to new vector list parsing/formatting functions

Fixes #148
Revises #149