Check precedences of metadata

[yangsophieee]

Addresses #60, checking precedence of metadata fields across various sections of metadata are working correctly (e.g. location > trait metadata)

• Added additional testing for checking precedence of metadata
• Fixed an error with dataset_test where I didn't realise the data variable already uses process_custom_code so it was applying it twice
• Removed old testing files
• Small formatting changes
• @ehwenk and I agree that trait metadata should probably take precedence over location metadata but we will leave this to a later date (trait metadata is read in via process_parse_data and location metadata replaces it afterwards in dataset_process around Line 141, so you'd have to move the location metadata part into process_parse_data, which will presumably require splitting location data into location properties vs the other variables and having them input at different times)

[dfalster]

Let me know once passing and I can review!

[yangsophieee]

@dfalster it's passing now! Thanks :)