Add wildcard support for paths() and streams() (fixes #54)

[JuhaKiili]

What

For a single Valohai input, we have .path() .paths() .stream() .streams().

This PR introduces a new parameter filter_path, which allows filtering a subset of all the files of the input, based on their file path. Wildcards are supported for both single and double notation.

Example: valohai.inputs("myinput").paths("folder/**/foo/*.jpg")

Why

If your input has multiple files, but you just want one (or subset).

Before this PR:

import fnmatch
for path in fnmatch.filter(valohai.inputs("myinput").paths(), "weights.pb"):
  print(path)

After this PR:

path = valohai.inputs("myinput").path("weights.pb"):

Also supports stuff like:

for image_path in valohai.inputs("myinput").paths("folder/**/foo/*.jpg"):
  print(image_path)

[ruksi]

lgtm, you might want to add the test mentioned in the TODO though?

[JuhaKiili]

Removed the TODO about mangled paths PR + handled all the merge conflicts with master.

[ruksi]

🥇