Add convenience methods

[yoshi74ls181]

Added convenience methods add_tag, backup_file, save_text, and save_dict to DDH5Writer.
They enable us to save things in the same directory as the ddh5 file.
Usage example:

import qcodes as qc
from plottr.data.datadict_storage import DataDict, DDH5Writer

station = qc.Station()
data = DataDict(
    time=dict(unit="s"),
    voltage=dict(unit="V", axes=["time"]),
)

with DDH5Writer(data) as writer:
    writer.add_tag(["tag1", "tag2"])
    writer.backup_file(__file__)
    writer.save_text("note.md", "some note")
    writer.save_dict("station_snapshot.json", station.snapshot())
    writer.add_data(
        time=[1, 2, 3],
        voltage=[4, 5, 6],
    )

[marcosfrenkel]

Looks good! I have opened issue #376 with possible improvements for these functions

[yoshi74ls181]

Thanks for the review! You can assign the issue to me, I'll work on it.