add option to show needed resources

[WolfgangFahl]

When experimenting with the OSM planet dataset i found out i did not have enough diskspace to run it.
It would be good if there would be an option showing how much RAM and diskspace a certain KG needs and maybe even check whether the host machine fulfills these prerequisites e.g. when a target device is given as a parameter.

[hannahbast]

@WolfgangFahl The value of STXXL_MEMORY should be an estimate of how much RAM the index build needs (the name is a misnomer). A rule of thumb for the amount of disk space needed is around 20 Bytes / triple (a bit more if the input has many distinct literals and IRIs, a bit less if it has relatively few of those). For example:

Dblp 0.5 B triples -> estimated size 10 GB -> the actual size is 9 GB
Wikidata 20 B triples -> estimated size 400 GB -> the actual size is 450 GB (because of the man distinct literals)
UniProt: 240 B triples -> estimated size 4.8 TB -> the actual size is 4.6 TB