Check config key

[yhmtsai]

This PR adds the check to verify the keys in config are allowed by predefined keys set.
typo in config is quite easy in the file because we do not have auto-complete for that
It avoids typo or misusage of parameters from user such that the configuring solver is not what users expect without notice.

Isome thoughts will be nice.
allowed_keys preparation: predefine it as early as possible or insert the key into the set close to the condition
I am predefining it as early as possible. but I think the second way might makes sense when the class has long list of keys like multigrid. It makes the manual type together and reduce the chance that missing deleting one of condition and allowed key. The first one is somehow easy to set with constructor not many lines of set.insert("key")

[MarcelKoch]

Suggestion: use the pnode to keep track of which keys of a map were accessed. If not all keys were accessed, then there are some invalid keys.
The pnode will also keep track of what is the set of allowed keys, since the parser function will test for all valid keys.

[upsj]

Implementation example:
Replace

if (auto& obj = config.get("max_iters"))

by

std::set<std::string> allowed_keys;
auto get_config_value = [](const char* key) {
    allowed_keys.insert(key);
    return config.get(key);
};
// ...
if (auto& obj = get_config_value("max_iters"))

The main goal is to specify every key name string only once, so we avoid accidental typos.

[yhmtsai]

It merge the allowed keys preparation with trying getting nodes now