adding button for fix_Lz option
updating fix_Lz_checkBox name
updating _ui_nanogen module with pyuic4 script
Merge branch 'nanogen_gui_dev' into dev
Adding new function `rezero_array`.

`rezero_array` sets elements of input array with absolute value
less than or equal to `epsilon` value.
adding `rezero_array` function to module namespace.
updating core module docstring
Removing un-implemented *_transforms functions.
Adding new function `axis_angle_from_rotation_matrix`.

axis_angle_from_rotation_matrix computes the rotation axis
and rotation angle from a rotation matrix.

Also adding new kwarg `axis` to functions that previously
relied on `rot_axis` to be more consistent with use of
variable names *axis* and *angle*.
adding todo docstring to axis_angle_from_rotation_matrix function
fix docstring directives
adding sigma_bond_angle attributes
Adding new custom iterator function `cyclic_pairs`.

`cyclic_pairs` generates a cyclic list of all pairs
(2-tuple length combinations) of elements from the
input `iterable`.
replacing itertools.combinations with new `cyclic_pairs` function
updating bond_angle list indexing to be consistent with bond_angle_pairs