Unable to save data file to output filepath that contains a % character

[jamesobutler]

As originally posted about in https://discourse.slicer.org/t/failures-saving-a-volume-to-path-with-sign/25657 and more specifically in
https://discourse.slicer.org/t/failures-saving-a-volume-to-path-with-sign/25657/6 thanks to @lassoan:

MetaIO supports One-Slice-Per-File Data Formats 1 and it uses the % character to specify the slice number in the filename format string.

Due to the special meaning of the % character, currently MetaIO does not allow using this chracter in the path. This limitation should either be clearly documented or the implementation should be made more sophisticated (to somehow differentiate the % that refers to per-slice filename generation from simple % occurrences in the path; or by allowing completely disabling this extremely rarely used one-slice-per-file feature).

As such, % used in the basename could possibly still be treated with the special meaning for the filename format string for One-Slice-Per-File functionality however, % used elsewhere in the filepath should be supported. Last resort would be to document this limitation.

The below code uses 3D Slicer to show that the mhd file is written for a volume output filepath that contains a "%", but the corresponding data file (.zraw) is not written.

import os
import SampleData

volume_node = SampleData.SampleDataLogic().downloadMRHead()

output_filepath = os.path.join(os.getenv("USERPROFILE"), "Downloads", "2%concentration", "MyVolume.mhd")
slicer.util.saveNode(volume_node, output_filepath)
print(f"{output_filepath} exists?: {os.path.exists(output_filepath)}")
raw_filepath = os.path.splitext(output_filepath)[0] + '.zraw'  # default Slicer saves it compressed hence zraw and not raw
print(f"{raw_filepath} exists?: {os.path.exists(raw_filepath)}")