Allow plus signs in 'to' and 'from' entities

[tsalo]

Closes #145.

I checked the following:

Before

import templateflow

print(templateflow.__version__)
# 25.0.1

print(len(templateflow.api.get('MNI152NLin6Asym', **{'from': 'MNIInfant', 'suffix': 'xfm'})))
# 11 (all of the cohort-specific MNIInfant transforms)

print(len(templateflow.api.get('MNI152NLin6Asym', **{'from': 'MNIInfant+10', 'suffix': 'xfm'})))
# 0 (no transforms collected)

After

import templateflow

print(templateflow.__version__)
# 25.1.0.dev2+g2ee563748

print(len(templateflow.api.get('MNI152NLin6Asym', **{'from': 'MNIInfant', 'suffix': 'xfm'})))
# 0 (no transforms collected)

print(templateflow.api.get('MNI152NLin6Asym', **{'from': 'MNIInfant+10', 'suffix': 'xfm'}))
# .../.cache/templateflow/tpl-MNI152NLin6Asym/tpl-MNI152NLin6Asym_from-MNIInfant+10_mode-image_xfm.h5

[tsalo]

@mgxd any chance you could release 25.0.2 today?

[mgxd]

just pushed it