An option to write sdp to directory instead of zip archive

[vasdommes]

For large problems, writing block files to sdp.zip is a bottleneck in sdp2input/pvm2sdp, see plots in #176.
It is done by a single process and cannot be parallelized.

The purpose of writing to zip archive instead of plain directory is to prevent running out of inodes error on some filesystems.

If this limit is not a problem and if time of writing sdp.zip is important (e.g. in Skydive, where one may call sdp2input after each several iterations of SDPB solver), it will be better to write to a directory. We can add e.g. an option --zip=false.
(NB: don't forget to validate all file sizes after writing!)

[davidsd]

Might there be some other kind of archive format besides zip that can be written to in parallel?

[vasdommes]

• pmp2sdp options: --zip flag instead of --zip=(false|true) option #183