Change eval methods for those expressions that may depend on multiple features

[elenatorro]

Context

.eval methods are used internally to evaluate a feature from an expression, for example, to get the "legend data" (through .getLegendData). Not all the methods need to evaluate a feature (for example, globalAggregation). However, there are some cases where there could be more than one feature to evaluate. For instance, in all the binary expressions (mul, add, sub...) evaluate two features.

To Do

We have to fix all the eval methods that require in two or more features to be evaluated. This will also imply to test those methods to build legends.

[elenatorro]

Note: it currently has low priority but this is a must if we want to include bivariate legends in CARTOframes.