Globals class

[GJHSimmons]

Supersedes #28.

We discussed that a globals class is required. This class should capture everything in a population that is not stored in current data structures (variables, parameters, windows, etc.). It should therefore contain:

• the variable time, or some other global independent variable(s);
• the functions available to be used in symbolic expressions. This should include functions already part of the sympy package, as well as custom functions, see Modular data-defined functions module #31;
• global parameters, such as temperature, pressure, even if they are read as functions of time from climate data.