Partial failure to obtain model-specific mappers in 2.7.0

[finnlindgren]

Model-defined mappers accessible via bru_get_mapper() methods are not detected as early as possible, leading a bug in handling of list inputs to prevent detection altogether, giving an error instead. Workaround

# Intended code
comp <- ~ field(list(space = ..., time = ...), model = mod)
# Workaround for 2.7.0:
comp <- ~ field(list(space = ..., time = ...), model = mod, mapper = bru_get_mapper(mod))

Fixed in a3bde51

[finnlindgren]

Solved by #193