Merge signed and unsigned integer types

[axelboc]

The goal is to split NumericType into two dtypes: IntegerType and FloatType, and to replace DTypeClass.Unsigned with a signed boolean in IntegerType.

I had first planned to introduce this change as part of adding support for big integers (cf. #1679 (comment)). At the time I wanted to introduce a new DType called BigIntegerType and it didn't really make sense to have this dtype together with NumericType... I'm now going back on this solution for reasons I'll explain soon, but I find that the NumericType refactoring still makes sense. IntegerType/FloatType brings us closer to the HDF5 types, and representing the sign as a boolean avoids relying on the DTypeClass enum's value too much in the metadata viewer (i.e. "Integer (unsigned)").

[axelboc]

We still keep a NumericType as that's still useful throughout the codebase.

[loichuder]

I am not awfully convinced by this. Personally, I have always valued descriptive values over flags.

Also

IntegerType/FloatType brings us closer to the HDF5 types

Is that really true ? As far as I can see, unsigned and signed integers are separate datatypes: https://support.hdfgroup.org/documentation/hdf5/latest/group___p_d_t_s_t_d.html

So unless this lays ground for another future refactoring, I need a little more to go along 😄

[axelboc]

I was referring to the H5T_class_t enum: https://support.hdfgroup.org/documentation/hdf5/latest/_h5_tpublic_8h.html#a071841985f647f69516dbe77d93167f2

[axelboc]

So the idea is to bring DTypeClass closer to H5T_class_t. The only two values that don't have equivalent are DTypeClass.Bool and DTypeClass.Complex, which are compound types with very specific, well-defined fields.

Another argument is that the class property is the discriminant property of the discriminated union type DType. That one of the interfaces in that union has class: DTypeClass.Integer | DTypeClass.Unsigned | DTypeClass.Float is a bit of a conflicting pattern.

(I'd like to split ArrayType into two, ArrayType and VLenType, for the same reasons.)

[loichuder]

Ok then 👍