provide hooks for user-defined types

[sbinet]

it should be possible for npyio.Write and npyio.Read to properly handle user-defined types, e.g. type MyMatrix struct {...}

probably by having said user-defined type implement an interface that allows to describe the internal data:

• a way to expose equivalent data to npyio.Header.Descr
• a way to expose the underlying data slice []T (where T is one of the scalar types that npyio supports)

Alternatively, have npyio expose npyio.Marshaler and npyio.Unmarshaler interfaces (which would then be tested for and used in npyio.Write and npyio.Read, respectively)
Tentatively:

type Type struct {
    Name    string // e.g. '<f8'
    Shape   []int
    Fortran bool
}

type Marshaler interface {
    MarshalNPy() (dtype Type, data []byte, err error)
}

type Unmarshaler interface {
    UnmarshalNPy(dtype Type, data []byte) error
}