[DualPorosity] (1/5) Add the RUNSPEC porosity model

[arturcastiel]

What this does

Adds PorosityModel, a value type for the continuum selection made in the RUNSPEC section:
single porosity (the default), dual porosity (DUALPORO) or dual permeability (DUALPERM),
together with the fracture permeability scaling option (NODPPM). Runspec holds one and
exposes it through Runspec::porosityModel(); no flags are added to Runspec itself.

The type answers the questions downstream code asks — does the run resolve two continua, does
it resolve flow between matrix cells, is fracture permeability scaling active — so the deck is
read in one place and every consumer takes the same answer. It follows the pattern of the other
RUNSPEC value types: constructor from the deck, serialization, equality, a serialization test
object.

Why

First of a short series adding a dual-continuum capability to Flow. A dual-continuum deck
declares twice the layers: the first half in K are matrix cells, the second half their
co-located fracture twins. Nothing can interpret that layout before the porosity model is known,
so this lands first.

Behavior impact

None. Flow still rejects every dual-continuum keyword; this adds the type, its accessor and the
per-cell coupling keyword's registration.

Testing

RunspecTests: the default model, each selection, DUALPERM taking precedence over
DUALPORO, the scaling option with and without NODPPM, equality. test_Serialization: the
round trip.

Requirements and limitations

Nothing consumes the type yet — the code that does follows in the next pull request.
Gravity-drainage options are deliberately not included; that work is separate and later.

[arturcastiel]

jenkins build this please

[bska]

I'd really prefer we not add more single-purpose bool flags directly to Runspec. Could you not create a separate structure for these flags and give it a more descriptive name?

[arturcastiel]

Agreed, and done. The three flags are gone from Runspec. The selection now lives in a value
type, PorosityModel (opm/input/eclipse/EclipseState/PorosityModel.hpp), which Runspec
holds and exposes through Runspec::porosityModel().

It records the model as an enum — SinglePorosity, DualPorosity, DualPermeability — so
"DUALPERM implies DUALPORO" is a property of the type rather than a comment, plus the NODPPM
option. The questions downstream code asks are methods on it: dualContinuum(),
dualPermeability() and fracturePermeabilityScalingActive(). It follows the pattern of the
other RUNSPEC value types (constructor from the deck, serializeOp, serializationTestObject,
operator==) and is covered in RunspecTests and test_Serialization.

The name follows the EGRID header, whose corresponding item distinguishes single porosity, dual
porosity and dual permeability. If you would prefer a different one, say so and I will rename.

EclipseGrid now reads the same type at construction, so these keywords are parsed in one
place. The later pull requests in the series are rebased onto this.

[arturcastiel]

jenkins build this please