Add option to disable combining units without clear operator

[nielstron]

Is your feature request related to a problem? Please describe.
Sometimes (see #135) normal words are interpreted as combined units i.e. "PayPal" = "petayear year petayear litre"

Describe the solution you'd like
An option in the parser function to disable the "" operator - only accept combined units with clear delimiter (like "*" or "/")

Describe alternatives you've considered
Further improve the detection of wrong interpretations.

Additional context
#135