[thorstenhater]

Absorb global parameters into the mechanism info by moving global parameters
from the parameter list into the derivation closure.

Example

density("pas", {{"e", 80}, {"g", 0.42}})
// becomes
density("pas/e=80", {{"g", 0.42}})

Includes a few refactorings around fvm_layout, like introducing flat_map<K, V> = vector<pair<K, V>>.
(Note, that flat_map probably wants some infrastructure from rangeutil later. For now it just establishes
a naming scheme and saves on typing. This is exists in maputil)