Define helper funtions for casting/saving metadata types

[JosePizarro3]

A very typical process during parsing is to read a float as a string from a file, and wanting to store it for a PropertyTypeAssignment which is of type REAL. Then, the end user needs to cast this down.

@CagtayFabry already pointed this out, so I will modify the __setattr__ to make it easier to use in these cases, and automatically cast these down for easier usability