Euphonic Calculations: Phonopy Gateway: Debug interpolation

[rebeccafair]

After reading into InterpolationData from Phonopy, the interpolation doesn't produce the correct frequencies/eigenvectors. Some things to check:

• Check all input units are converted correctly
• Check force constants is reshaped correctly
• Compare Phonopy NaCl calculation with CASTEP NaCl calculation

Related to #53

Sub issue of #12

[rebeccafair]

The frequencies weren't being reproduced correctly as the force constants matrix wasn't being reshaped correctly. This has now been fixed.