Multiple structures uploader

[yakutovicha]

I've been recently reviewing tools to select and upload several structures at one shot. Unfortunately, even though it is considered useful, neither fileupload nor ipywidgets do currently provide such multi upload functionality. Then I looked closely to those packages trying to understand if this was somehow considered.

First I looked at fileuploader package that we use quite extensively. Unfortunately, even though the possibility to upload multiple files was considered useful and people actually started working on that, it looks to me that no significant progress was made in this direction.

Second I looked at ipywidgets and discovered that there is a long-standing discussion about adding both: file upload and multiple file upload features to the package. A PR have been created and is still under review process. So we should wait until it will be finally approved.

Therefore, for the moment I will go standard way: uploading an archive file that contains structures, unpack them and store one by one in AiiDA.

[ltalirz]

Added in e3002c5