population_template: replace whitespace in unique filename

[maxpietsch]

Fix to population_template which currently does not handle whitespace characters in the unique identifier parts of the input files. This fix replaces whitespace with '_' as internally, population_template requires filenames in its scratch directory to be whitespace free.

Likely fix to https://community.mrtrix.org/t/fba-population-template-error-uid-contains-whitespace/4229/4

➜  mt_reg_test ls -l odfA3
total 3952
-rw-r--r--  1 mp  staff  699956 26 Oct 14:32 odf 0.mif.gz
-rw-r--r--  1 mp  staff  642369 26 Oct 14:32 odf 2.mif.gz
-rw-r--r--  1 mp  staff  678072 26 Oct 14:33 odf subject 1.mif.gz

➜  mt_reg_test population_template odfA3 out.mif -scratch /tmp/scratch -debug
population_template: [DEBUG] path.from_user() (from population_template:291): out.mif -> /Users/mp/data/mt_reg_test/out.mif
population_template: [DEBUG] execute() (from app.py:197): 	contrast: ., template: /Users/mp/data/mt_reg_test/out.mif, suffix: _c0

population_template: Generating a population-average template from 3 input images
population_template: [WARNING] no masks input. Use input masks to reduce computation time and improve robustness
population_template: common_postfix: ['.mif.gz']
population_template: common_prefix: ['odf ']
population_template: xcontrast_xsubject_pre_postfix: ['odf ', '']
population_template: [DEBUG] path_to_file_uid() (from population_template:519): UID mapping: "odf 0.mif.gz" --> "0"
population_template: [DEBUG] path_to_file_uid() (from population_template:519): UID mapping: "odf 2.mif.gz" --> "2"
population_template: [DEBUG] path_to_file_uid() (from population_template:519): UID mapping: "odf subject 1.mif.gz" --> "subject_1"
population_template: uids by contrast:[['0', '2', 'subject_1']]
population_template: [DEBUG] path.from_user() (from population_template:532): odfA3 -> /Users/mp/data/mt_reg_test/odfA3
population_template: [DEBUG] path.from_user() (from population_template:532): odfA3 -> /Users/mp/data/mt_reg_test/odfA3
population_template: [DEBUG] path.from_user() (from population_template:532): odfA3 -> /Users/mp/data/mt_reg_test/odfA3
population_template: performing leave-one-out registration