Add methods to remove fields

[hongyuanjia]

Currently there is no directly way to remove fields. Probably can add a new parameter .remove in Idf$set()

Take the construction below as an exmaple:

Construction,
    Const,           !- Name
    Layer1,          !- Outside Layer
    Layer2,          !- Layer 2
    Lyaer3;          !- Layer 3

If we want to remove Layer2 and move Layer3 to field Layer 2, we can do

idf$set(Const = list(Layer_2 = NULL), .remove = TRUE)

The results would be

Construction,
    Const,           !- Name
    Layer1,          !- Outside Layer
    Layer3,          !- Layer 2
    ;                !- Layer 3