Change all symbols to include the assumption real=True

[tjstienstra]

All symbols and dynamicsymbols which are currently being used in BRiM are real. This would be useful to automatically specify when creating a model, as that simplifies results, e.g. sqrt(a**2)=a.

There are two basic options of implementing this:

1. Create a real_symbols and real_dynamicsymbols function in the utilities to be used throughout BRiM, while using dynamicsymbols._t when aquiring the time symbol.
2. Update all symbols and dynamicsymbols calls to specify real=True.