added required frame_convention argument to cell parameter conversions

[bobmyhill]

This PR makes the functions utils.unitcell.cell_parameters_to_vectors() and utils.unitcell.vectors_to_cell_parameters() more flexible by adding an additional argument frame_convention. This required argument takes a list of three integers that define the order of axes and vectors.

The convention dictates that the c[0]th cell vector is colinear with the c[0]th-axis, the c[1]th cell vector is perpendicular to the c[2]th-axis, and the c[2]th cell vector is defined to give a right-handed coordinate system. In common crystallographic shorthand, $x_{c_0}$ // $a_{c_0}$, $x_{c_2}$ // $a_{c_2}^*$.

The new argument is set to be required, as there is no "default" option for non-orthotropic materials - different conventions are more suitable for different materials.