pass settings from -Xjuliacall_ to julia

[dpinol]

added support for --threads, --procs, --min-optlevel

Fixes #184

[cjdoris]

Great PR, thank you! Most of the options seem to work great, but some require special handling:

• opt_bindir translates to --home not --bindir. Could rename it opt_home?
• opt_bindir and opt_sysimage are parsed by jl_parse_opts but have no further effect. I think the call to jl_init__threading needs to be replaced by jl_init_with_image__threading.
• opt_procs also has no effect, it's handled separately in Base here (but is only called later in the loading process than jl_init): https://github.com/JuliaLang/julia/blob/762561c3057b4bd541d183ca1d1b70b9945bff22/base/client.jl#L245-L254

The first one should be fixed in this PR. I'm happy to leave the others for separate PRs (and not document them yet) - it's up to you.

Speaking of bindir, there is the Julia env var JULIA_BINDIR and the command line option --home, hence the naming discrepancy in the first bullet point. It seems like JuliaCall should have similar behaviour (PYTHON_JULIACALL_BINDIR and -X juliacall_home) to be consistent with Julia. Opinions?

[cjdoris]

I've just pushed a change to the main branch which, when merged with this PR, means that opt_procs works.

[thautwarm]

opt_bindir and opt_sysimage are parsed by jl_parse_opts but have no further effect. I think the call to jl_init__threading needs to be replaced by jl_init_with_image__threading.

After merging this, I could make a PR for supporting opt_bindir and opt_sysimage.

[dpinol]

thank you guys!

[cjdoris]

I've merged this PR and made a few tweaks, to get the bindir/home option working. I've also changed the -X options to use --separated words instead of _. And they are documented here: https://cjdoris.github.io/PythonCall.jl/dev/juliacall/#julia-config

A PR to support bindir and sysimage would be great.