Inconsistent naming of `salt_score` v `sodium_score`

[Sparrow0hawk]

After merging #41 a new error has been introduced when calling NPMAssess:

Score columns not detected. Please ensure you have called NPMScore on your data 

This is because NPMAssess expects a sodium_score column which isn't present because NPMScore spits out a salt_score column.

• [ ] Change NPMAssess to expect salt_score column
• [ ] Update integration test to catch this error