Quantity.is_compatible reports undefined quantities as incompatible

[jpalm3r]

Quantity.is_compatible (src/modelskill/quantity.py:61) is meant to let an undefined quantity compare cleanly against anything. It does not, because it tests for the wrong sentinel.

from modelskill import Quantity

u = Quantity.undefined()
wl = Quantity(name="Water Level", unit="meter")

u                                                     # Quantity(name='', unit='')
wl.is_compatible(u)                                   # False  <- expected True
u.is_compatible(wl)                                   # False  <- expected True
wl.is_compatible(Quantity("Undefined", "Undefined"))  # True

Two separate problems:

1. Wrong undefined sentinel. The method checks self.name == "Undefined", but Quantity.undefined() returns Quantity(name="", unit=""). Only the literal string "Undefined" is recognised, which is not what the constructor produces.

2. Unit-strict. Beyond the undefined case it falls back to self == other, so two quantities with the same name but different units never agree:

Quantity("Pressure", "").is_compatible(Quantity("Pressure", "MetresWater"))  # False

This case is not hypothetical. Model results read from res1d and EPANET files carry a quantity name but no unit — mikeio1d exposes quantities as a plain list of names — so a network model result is legitimately Pressure [] while the matching observation is Pressure [MetresWater].

Why it has gone unnoticed

is_compatible is called from nowhere in src/ or tests/. Which points at the larger gap: match() never compares an observation's quantity with the model's at all.

# observation labelled Water Level, model labelled Discharge
cmp = ms.match(obs, mr)
cmp.n_points    # 10
cmp.quantity    # Water Level [meter]  <- the model's quantity is ignored entirely
cmp.skill()     # scores it happily

The Comparer adopts the observation's quantity and never looks at the model's, so a mismatched comparison ends up mislabelled rather than merely wrong.

Suggested fix

• Treat both "" and the legacy literal "Undefined" as undefined.
• Compare names on a normalised key (lowercased, non-alphanumerics stripped) so "WaterLevel", "Water Level" and "water_level" agree. Names arrive from different vocabularies (EUM item names, CF long_name), and trivial spelling differences should not count as conflicts.
• Compare units only when both are non-empty, normalised the same way. unit_display_name (src/modelskill/obs.py:768) already maps meter→m, second→s, degree→° and can be reused.
• Then wire it into match() behind a check_quantity="error"|"warn"|"ignore" argument, matching the vocabulary of the existing obs_no_overlap. The default deserves a deliberate decision: strict catches the dangerous case, but risks false positives where the same physical quantity is named differently on the two sides.

The two rendered docstring examples on is_compatible keep their current results under this rule, so the generated docs are unaffected.