Allow to generate only data files for PostgreSQL

[jmarton]

In order to allow for easier easy experimenting, add an option to the PostgreSQL converter (load.sh):

• PG_LOAD_TO_DB: controls whether we want to do or skip database loading phase. Possible values are:
  • load: loads to the database, as one might expect. This is the default.
  • skip: skip loading to the database.

[szarnyasg]

We could use a similar approach to the Cypher loading script, which currently works as follows:
https://github.com/ldbc/ldbc_snb_implementations/blob/e5ec590e0fb18d7f2a5218b10561a48a619a5bda/cypher/load-scripts/load-in-one-step.sh#L1-L3

[szarnyasg]

Fixed by PR #125.