Allow optionals with a default value to be exclused from export

[aglavic]

Make the output YAML more concise by removing optional arguments not only when they are None but also when they have the default value. The particular application is in the model language wher the global ModelParameters always prints all value, even if the user just wanted to overwrite a single one.

[bmaranville]

Can you provide a simple script for reproducing the problem that this PR addresses? Can it be shown with one of the tests?

[aglavic]

Thanks @bmaranville, good point. This way I already found a bug that this was only done in to_dict, not in to_yaml. I've now added a testcase.

Just to illustrate, if you run the following:

from orsopy.fileio import model_language as ml
import yaml
print(ml.ModelParameters.from_dict(yaml.safe_load('roughness: {magnitude: 0.7, unit: nm}')).to_yaml())

The output before would be:

roughness: {magnitude: 0.7, unit: nm}
length_unit: nm
mass_density_unit: g/cm^3
number_density_unit: 1/nm^3
sld_unit: 1/angstrom^2
magnetic_moment_unit: muB
slice_resolution: {magnitude: 1.0, unit: nm}
default_solvent:
  formula: H2O
  mass_density: {magnitude: 1.0, unit: g/cm^3}

And now is:

roughness: {magnitude: 0.7, unit: nm}