The timedata class function save() should return a path object and not a string

[ehsteve]

Our standard is to use paths everywhere instead of strings to have more portable code.

The title is self explanatory. This would make it easier to test whether a file was created by using the function is_file for example.

[Alrobbertz]

I have completed and merged the required changes in Core through #117 .

I have shown that the changes work for the instrument repositories though EEA HERMES-SOC/hermes_eea#41. I will replicate these changes for the other instrument repos and merge them all at the same time.

[Alrobbertz]

Closed with

◦ #117
◦ HERMES-SOC/hermes_eea#41
◦ HERMES-SOC/hermes_merit#22
◦ HERMES-SOC/hermes_spani#27
◦ HERMES-SOC/hermes_nemisis#25