Option to request data availability instead of download in mass_downloader

[icui]

Problem

Some data centers offer additional options to request data (e.g. BREQ_FAST from SAGE). mass_downloader.download() function offers a convenient way to request data availability, but it directly starts downloading after that. It would be helpful to offer an option to only return the available stations so that users can generate request files for their special needs.

Proposed solution

return helper or station list after helper.prepare_mseed_download() in mass_downloader.download() function.