A simple implementation of variables for the pharmacological expansion

[gkarfakis19]

Summary

Features "Implements basic variables for the pharmacological expansion"

Purpose of change

Adds some of the basic variables required to make #53152 work.

Describe the solution

As of yet I've implemented the code scaffolding required for the variables.
Additionally, I've used code from the existing heart_rate function to create some simple modifiers affecting heart rate.
Balancing WIP.

Describe alternatives you've considered

Not doing this.

Testing

Some basic testing was done to ensure all functions worked properly, using debugmsg() to display the values.

Additional context

Should the Heart Rate index calculations be as complicated? The original issue mentioned that these variables should initially be influenced solely by the consumption of drugs, but the presence of a well thought out heart_rate function in the code that depended on many factors made me reconsider this, as adapting it to work for heart rate index made sense. I'm especially looking for @I-am-Erk's thoughts on this.

[I-am-Erk]

@Venera3 will want to look at this with me

[gkarfakis19]

How much do you think should fit into this single PR? As far I can see this can probably be merged after sorting out the heart rate modifiers, and separate PRs could be created with the harmful effects for low/high values of these variables as well as the effects of medication/drugs on them.

[fungamer2-2]

Seems interesting! This will definitely make things more realistic