[WIP] Support parsing phonon results in the .castep file

[aplowman]

Working to resolve #2.

This might be a useful resource: https://www.tcm.phy.cam.ac.uk/castep/Phonons_Guide/Castep_Phononsch2.html

[aplowman]

Support parsing:

• species
• unit cell
• lattice parameters
• coordinates of elements
• point group
• which modes are raman active (and at which frequencies)
• which modes are IR active (and at which frequencies)
• Optical and DC permittivity
• optical and static polarisability
• born effective charge for all elements