extforce-convert: initialverticaltemperatureprofile (and salinity) should not require and write field *value* and *operand*

[JulienGroenenboom]

Describe the bug
When converting quantity's initialverticaltemperatureprofile and initialverticalsalinityprofile, HYDROLIB-core gives an validation error:
1 validation error for InitialField. value should be provided when datafiletype is polygon (type=value_error)

As the relevant values of the quantities are in the *.pli, the value should not be required. In case I do provide a dummy value, the conversion works. However, the value is now filled in the *.ini file, but for these specific quantities, fields value should not be required. Furthermore, field operand is not needed/used, but this does not lead to an error.

This has recently been improved in the FM-kernel (UNST-9416) by adding .and. method == METHOD_CONSTANT, see below. I suggest adjusting HYDROLIB-core accordingly.

if (filetype == inside_polygon .and. method == METHOD_CONSTANT) then
    read VALUE
end if

Additional request:
Change description of interpolationMethod to: Type of (spatial) horizontal interpolation

To Reproduce
Steps to reproduce the behavior:

1. cd p:\dflowfm\users\veenstra\extforce-convert\initialverticalprofile\input\

2. extforce-convert --mdufile model.mdu

3. Observe the validation error

4. cd p:\dflowfm\users\veenstra\extforce-convert\initialverticalprofile\input_including_dummy_value\

5. The conversion works, but value should not be written/required.

Version info (please complete the following information):

• Windows
• dfm_tools_env version from DSD-INT (Nov 2025)

Additional context
Corresponding kernel issue: https://issuetracker.deltares.nl/browse/UNST-9416