Check if the session is interactive before prompting for user input

[BSchilperoort]

At some points (e.g. when checking for existing files), users are prompted with a Y/N question. We should add a check if the user can actually answer the question, or if the script is called by another script.

io.isatty() can be used for this.

We can also then disable the CDSAPI's progress bar for non-interactive cases.

(suggestion by @sverhoeven )

[BSchilperoort]

Implemented in #152