allow tool path overrides via concore.tools config and env vars

[avinxshKD]

tool paths (g++, iverilog, python3, matlab, octave) can now be overridden via:

• concore.tools config file (KEY=value format)
• env vars (CONCORE_CPPWIN, CONCORE_CPPEXE, etc.)

priority: config file > env var > hardcoded default

defaults unchanged (g++, iverilog, python3, matlab, octave) so existing setups keep working

• added 6 tests for the config parser

• updated README with config docs

fixes #247

[avinxshKD]

@pradeeban pls look into it, Ive also updated README as asked, thanks

[pradeeban]

Lots of line changes that have no change in mkconcore.py. Perhaps, line ending/linebreak change? Can you recreate the PR with minimal number of line changes? That will avoid merge conflicts and help with maintenance.

[avinxshKD]

@pradeeban I recreated the branch from upstream/dev and force-pushed a clean update with only minimal, relevant changes, pls let me know if any adjustments needed thanks