Enable more concurrency via the Makefile

[martin-ueding]

Most calls with pvcall are parallelized. Some other calls use serial = TRUE but then have boot::tsboot with its own parallelism. In general one can say that all the things from the paramval directory are to be run serial, with make -j 1. The rendering of the RMarkdown documents is single-threaded and could be parallelized with make -j $(nproc).

It seems that this mixture is not supported by GNU make, see this Stack Overflow question.

I currently see the following options:

1. Use a second Makefile with the RMarkdown targets and call that with make -j $(nproc). This will improve throughput, but the dependencies have all to be satisfied before this can be done. Dependency handling inter-Makefile sounds like needless pain.

2. Not use GNU make but instead see whether Ninja-Build has this feature. Makefile generation should still be retained because system out of our control very likely do not have Ninja-Build installed.

3. Use a single Makefile and have processes communicate with some sort of semaphore to prevent oversubscription. That is likely going to end in deadlocks rather quickly.