Change temporary directory every 10.000+ active files

[SSoelvsten]

On the Grendel cluster, we have created a head-ache for the sysadmins by creating 175.000 concurrent files in the temporary folder (presumably model checking a system of about 20.000+ transitions). We ought to change (sub)directory every 10.000 files.

Tasks

• [ ] Each subdirectory is a mere number 0, 1, 2, ... in the given tmp_path.
• [ ] When creating a file, keep track of whether this is file number 10.000 in the current directory with an (atomic) counter. If so, increment the subdirectory number for the next set of files and reset the counter.

[SSoelvsten]

We may as well set this up to be the basis for our own file_manager (for #199 , #679 , #678 , and so on). In fact, we may want to flip the problem on its head - leave TPIE to place unnamed temporary files (sorter, priority queue, and arc-based results) in its temporary folder while we create a second temporary folder for all of Adiar's temporary BDDs and ZDDs.

Tasks

• [ ] Allow a file to be temporary even if given path.
• [ ] Obtain (temporary) files from a global file manager.
  • [ ] If we want to create fewer temporary directories: keep track of the number of files in each subdirectory (presumably this can be done with a single vector of integers). In the destructor of a file, decrement said counter. Now a new subdirectory is only created if truly needed.