feat: Support the old style external forcings file

Refs: #369

docs/reference/extold.md

@@ -0,0 +1,7 @@
+# External forcings file
+The external forcing .ext file contains the forcing data for a [D-Flow FM](glossary.md#d-flow-fm) model.
+This includes open boundaries, lateral discharges and meteorological forcings.
+The documentation below only concerns the 'old' format (`ExtForceFile` in the MDU file).
+
+## Model
+::: hydrolib.core.dflowfm.extold.models

hydrolib/core/dflowfm/__init__.py

@@ -2,6 +2,7 @@
 from .common import *
 from .crosssection import *
 from .ext import *
+from .extold import *
 from .friction import *
 from .gui import *
 from .inifield import *

hydrolib/core/dflowfm/common/__init__.py

@@ -1,3 +1,3 @@
-from .models import LocationType
+from .models import LocationType, Operand
 
-__all__ = ["LocationType"]
+__all__ = ["LocationType", "Operand"]

hydrolib/core/dflowfm/common/models.py

@@ -15,3 +15,23 @@ class LocationType(str, Enum):
 
     all = "all"
     """str: Denotes that both 1D and 2D locations may be selected."""
+
+
+class Operand(str, Enum):
+    """
+    Enum class containing the valid values for the operand
+    attribute in several subclasses of AbstractIniField and ExtOldForcing.
+    """
+
+    override = "O"
+    """Existing values are overwritten with the provided values."""
+    append = "A"
+    """Provided values are used where existing values are missing."""
+    add = "+"
+    """Existing values are summed with the provided values."""
+    mult = "*"
+    """Existing values are multiplied with the provided values."""
+    max = "X"
+    """The maximum values of the existing values and provided values are used."""
+    min = "N"
+    """The minimum values of the existing values and provided values are used."""

hydrolib/core/dflowfm/extold/__init__.py

@@ -0,0 +1,19 @@
+from .models import (
+    ExtOldExtrapolationMethod,
+    ExtOldFileType,
+    ExtOldForcing,
+    ExtOldMethod,
+    ExtOldModel,
+    ExtOldQuantity,
+    ExtOldTracerQuantity,
+)
+
+__all__ = [
+    "ExtOldExtrapolationMethod",
+    "ExtOldForcing",
+    "ExtOldModel",
+    "ExtOldQuantity",
+    "ExtOldFileType",
+    "ExtOldMethod",
+    "ExtOldTracerQuantity",
+]

hydrolib/core/dflowfm/extold/common_io.py

@@ -0,0 +1,23 @@
+from typing import List
+
+ORDERED_FORCING_FIELDS: List[str] = [
+    "QUANTITY",
+    "FILENAME",
+    "VARNAME",
+    "SOURCEMASK",
+    "FILETYPE",
+    "METHOD",
+    "EXTRAPOLATION_METHOD",
+    "MAXSEARCHRADIUS",
+    "OPERAND",
+    "VALUE",
+    "FACTOR",
+    "IFRCTYP",
+    "AVERAGINGTYPE",
+    "RELATIVESEARCHCELLSIZE",
+    "EXTRAPOLTOL",
+    "PERCENTILEMINMAX",
+    "AREA",
+    "NUMMIN",
+]
+"""List[str]: List of the ordered fields names in a forcing block."""