convert kwargs to Dict

[federicazanca]

There are some inputs that might be not worth saving in the provenance graph (e.g. the write-kwargs), but that we still want to have control over, so we need to be able to pass them through aiida data types. Save them as Dict rather than every parameters as Str.
More specifically, at the moment we have model and default_dtype as part of calc-kwargs that definitely need to be saved as data types.
write-kwargs only gets the filename, and I don't even pass read-kwargs.
All the other possible parameters at the moment cannot be controlled. They should be, but in a Dict. Same for the future parameters in MD etc.

for example at the moment for geom opt I have all these inputs and outputs (some are in common with single point). It's fine to save them all but we could also reorganise them
Inputs PK Type

---

architecture 1609 Str
code 2 InstalledCode
device 1613 Str
fully_opt 1614 Bool
log_filename 1616 Str
max_force 1619 Float
model 1612 ModelData
opt_kwargs 1621 Dict
precision 1610 Str
steps 1620 Int
structure 1611 StructureData
traj 1617 Str
vectors_only 1618 Bool
xyz_output_name 1615 Str

Outputs PK Type

---

final_structure 1631 StructureData
log_output 1625 SinglefileData
remote_folder 1623 RemoteData
results_dict 1628 Dict
retrieved 1624 FolderData
std_output 1627 SinglefileData
traj_file 1629 SinglefileData
traj_output 1630 TrajectoryData
xyz_output 1626 SinglefileData