Declare text and numeric types for char arrays

[jmw182]

char arrays have always been ambiguous since they may contain either numerical values or strings. Currently these are always read as numerical values, and the clients are responsible for recasting to a string.

It would be great to declare the intended high-level type of ambiguous low-level types such as char arrays. This could be part of the info contained in an "extended header" along with other new features. Then the client could automatically cast char arrays containing text to a string type (similar to casting numeric arrays to numpy arrays #29).