[MPM] Allow "use_input_model_part" in "model_import_settings" ProjectParameters.json

[ncrescenzio]

📝 Description
Allow ProjectParameters.json to contain

 "model_import_settings"           : {
     "input_type" : "use_input_model_part"
 },
 "grid_model_import_settings"      : {
     "input_type" : "use_input_model_part"
 },

• The value "use_input_model_part" is used when using modelers to laod MDPA files.
• Up to now it was not possible to use "use_input_model_part" because an exception was raised if "input_type" was different from "mdpa"/"rest".
• The check on the value of "input_type" is still performed in _ImportModelPart, so an exception is still raised if the value is not "mdpa"/"rest"/"use_input_model_part".

Related to KratosMultiphysics/GiDInterface#1004 and KratosMultiphysics/GiDInterface#1026.

FYI @jginternational

[jginternational]

Tested against KratosMultiphysics/GiDInterface#1026

It works

[jginternational]

Once you merge this PR, I'll merge the GUI PR