Add support for attributes

[perkinsb1024]

As far as I can tell, this library does not include support for reading/writing/deleting attributes ( https://www.hdfgroup.org/HDF5/doc1.6/UG/13_Attributes.html )

While it's not clean or complete, I was able to create attributes of type Double on groups by adding this code to Group.swift:

    /// Create an attribute
    public func createAttribute(name: String) -> hid_t {
        let dataspace = Dataspace.init(dims: [1, 1])
        let datatype = Datatype(type: Double.self)!
        let attributeId = name.withCString{
            return H5Acreate2(id, $0, datatype.id, dataspace.id, 0, 0)
        }
        return attributeId
    }

    /// Write an attribute
    public func writeAttribute(attributeId: hid_t, value: Double) {
        let datatype = Datatype(type: Double.self)!
        var cValue = CDouble(value)
        H5Awrite(attributeId, datatype.id, &cValue)
    }

    /// Close an attribute
    public func closeAttribute(attributeId: hid_t) {
        H5Aclose(attributeId)
    }

[alejandro-isaza]

Done!