Get rid of nested "netcdf.variable.name" entry

[JoostBuitink]

Right now, if a user wants to use a scale and/or offset to a variable, they need to specify the name of the NetCDF layer using "netcdf.variable.name". This creates a nested entry, which is not really necessary. We can rename this with netcdf_variable_name. See below for an example of the current implementation and a suggested new implementation.

Note that this also requires hydromt_wflow developments, to support correctly translating v0.x tomls to v1.0 tomls.

Current

[input.forcing.atmosphere_water__precipitation_volume_flux]
netcdf.variable.name = "P"
scale = 1.5
offset = 0.5

# or when not using scale/offset
[input.forcing]
atmosphere_water__precipitation_volume_flux = "P"

New?

[input.forcing.atmosphere_water__precipitation_volume_flux]
netcdf_variable_name = "P"
scale = 1.5
offset = 0.5

# or when not using scale/offset
[input.forcing]
atmosphere_water__precipitation_volume_flux = "P"

[JoostBuitink]

Ideally, perform these changes once #689 is merged, to avoid merge conflicts