make `DataViewer`'s HDF5 open mode/locking configurable

[tj-eldridge]

Related ewoksxas issue: https://gitlab.esrf.fr/workflow/ewoksapps/ewoksxas/-/work_items/39

Where

ewoksorange.gui.widgets.data_viewer.DataViewer
(ewoksorange/gui/widgets/data_viewer.py), method updateFile.

Verified against the installed ewoksorange==5.0.1:

def updateFile(self, filename):
    if not os.path.exists(filename):
        return
    h5file = self.__getFileObject(filename)
    if h5file is not None:
        self.__refreshAction.trigger()
        return
    self.closeFile(filename)
    model = self.__treeview.findHdf5TreeModel()
    h5file = h5py.File(filename, mode="a")   # <-- hardcoded
    ...

mode="a" is hardcoded with no way for a project using DataViewer to override
it, and no locking= argument is passed either, so the default (locking
enabled) always applies.

Problem

Projects embedding DataViewer for read-only browsing (e.g. inspecting an
HDF5 file that may be actively written by another process) have no way to:

1. Open the file read-only instead of append/read-write.
2. Disable HDF5 file locking to avoid contending with another process's lock
   on the same file.

Proposed change

Add configurability at construction time, with the current behaviour kept as
the default so nothing else in ewoksorange/downstream breaks:

class DataViewer(qt.QWidget):
    def __init__(self, parent, open_mode: str = "a", locking: bool = True):
        ...
        self.open_mode = open_mode
        self.locking = locking

And use it in updateFile:

h5file = h5py.File(filename, mode=self.open_mode, locking=self.locking)

Notes for implementation

• Default values (open_mode="a", locking=True) preserve current behaviour
  for existing callers.
• Setting open_mode="r" together with locking=False is the case we need
  downstream (ewoksxas), to browse a file without contending with another
  process's write lock.

Acceptance criteria

• DataViewer(parent) behaves exactly as it does today (append mode, locking
  enabled).
• DataViewer(parent, open_mode="r", locking=False) opens files read-only
  without taking an HDF5 file lock, and can browse a file already open
  (read or write) in another process.
• self.open_mode / self.locking are read by updateFile (and any other
  call site currently hardcoding mode="a", if more exist) rather than the
  literal being duplicated.