rework equation tags

As equation tags are used more and more often throughout the code, the initial structure has become cumbersome. Currently, they are stored as vector<pair<int, pair<string, string>>> equation_tags;. Rework this to make it easier to search for tags associated with a particular equation. Could store it as vector<map<string, string>>, vector<set<pair<string,string>>> or could even make an equation tags class....