Extending semantics of BLOSC_NTHREADS and default value

[werner17]

Some suggestions for the BLOSC_NTHREADS environment variable: It could be useful to have a zero or negative value to mean the number of cpu's not to use. So a value of 0 would mean to use all available CPU's, a value of -1 would mean to leave off one processor etc.

Currently its a bit workarounding that an application needs to find the number of threads itself and set it as environment variable, and it needs to do so before any BLOSC code might be possibly invoked. The default should be 0 then instead of 1, to use all available power . Currently it requires setting the environment variable to enable multithreading at all, but for a binary that you just give someone, that's too technical for all users (particularly windows-click-users).

A further option could also be support for values like BLOSC_NTHREADS=50% to always mean using half of the available CPU's only.