When using NLOHMANN_DEFINE_TYPE_INTRUSIVE, Stop the exception if dont't have the key-value pairs

[i-curve]

Hi, I like this library very much and I use it a lot.

When I use the NLOHMANN_DEFINE_TYPE_INTRUSIVE macro, I encountered some differences in usage habits.
the NLOHMANN_DEFINE_TYPE_INTRUSIVE implements to_json and from_json functions.
The detail of from_json will use NLOHMANN_JSON_FROM
the defination of NLOHMANN_JSON_FROM is

#define NLOHMANN_JSON_FROM(v1) nlohmann_json_j.at(#v1).get_to(nlohmann_json_t.v1);

if the json string don't have the key, it will throw an exception and terminal the program.
In my use case, if the key-value pair doesn't exist, you should ignore it instead of terminating the program.so I think it would be better to ignore the key-value pairs:

#define NLOHMANN_JSON_FROM(v1) if(nlohmann_json_j.contains(#v1)) nlohmann_json_j.at(#v1).get_to(nlohmann_json_t.v1);

@nlohmann Which method is better?

[gregmarr]

See NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT

[i-curve]

I never knew about this method and didn't see it in the readme.
hope add the NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT and .value() method in the readme

[gregmarr]

Now #4260 for the macros.