Line continuation in quoted graph string produces an error

[elliotfontaine]

Description

This only happens with single-quoted graph strings (not unquoted or with triple-quoting)

Reproducible Example

[scheduling]
    [[graph]]
        R1/+P3 = "quantify => \
        compute_fluxes"

[elliotfontaine]

Actually, the bug also affects single-quoted setting, unquoted graph string, and unquoted setting