Feature Request - support for real valued recordings

[777arc]

Everything is represented under the hood as complex float 32, but it's not a super quick addition because there's some code involved in fetching bytes from a file that may need *2 removed for real-valued signals.

Also every datatype needs a "c" or "r" so have the parser spit out an error if it finds a recording without either, and remove ones like i8 and u8 from list

From SigMF:

dataset-format = (real / complex) ((type endianness) / byte)

real = "r"
complex = "c"

type = floating-point / signed-integer / unsigned-integer
floating-point = "f32" / "f64"
signed-integer = "i32" / "i16"
unsigned-integer = "u32" / "u16"

endianness = little-endian / big-endian
little-endian = "_le"
big-endian = "_be"

byte = "i8" / "u8"

So, for example, the string "cf32_le" specifies "complex 32-bit floating-point samples stored in little-endian", the string "ru16_be" specifies "real unsigned 16-bit samples stored in big-endian", and the string "cu8" specifies "complex unsigned byte".