Document NIPYPE_NO_MATLAB

[effigies]

Nipype respects the NIPYPE_NO_MATLAB environment variable, which preempts any attempt to find a MATLAB installation. This is undocumented behavior.

Where would be a good place to list all environment variables we respect? (I think this is the only Nipype-specific one.)

[effigies]

Other environment variables:

• NIPYPE_NO_ET - Disables etelemetry for nipype specifically, but not in general
• NIPYPE_CONFIG_DIR - Alternative to ~/.nipype to look for a global configuration file