Deprecate hybrids

[gdementen]

groupby(agegroup, expr=avg(equi_income))
'MethodSymbol' object is not iterable 

[gdementen]

However, hybrids are the only way to make global temporaries. Global temporaries are only meant as fields which are not saved at the end of each period, so that will make solving issue #58 more pressing. However, given that I doubt many people know how to make global temporaries, we could deprecate hybrids anyway and have them use fields instead until #58 is done.