Optimisation recommendation - order of switches

[jimc101]

In several places in the code, a selection is made between different values for switches (e.g. the choice of the formulation used to compute oxygen saturation concentrations). As a basic optimisation, it is best to put the most commonly used option first in order to avoid the program performing one or more integer comparisons that always fail before eventually hitting the correct option. In ERSEM, this is often not done. For example, the first choice for oxygen saturation concentrations is "legacy ERSEM", which is not the option specified in the example yaml files.