FIX: Lingering temporary directories

[mgxd]

This is a spin-off of #466 and a continuation of #485

After first confirming the problem with a failing test, this PR:

• Removes custom temporary directory object from various function signatures
• Replaces temporary directory creation with context managers (where possible)
• Simplifies and moves TempDirs object into a dedicated module

[yarikoptic]

the fail

=================================== FAILURES ===================================
____________________________ test_smoke_convertall _____________________________
heudiconv/tests/test_heuristics.py:42: in test_smoke_convertall
    assert not list(Path(tempfile.gettempdir()).glob('heudiconv*'))
E   AssertionError: assert not [PosixPath('/tmp/heudiconvDCM42wzdx58'), PosixPath('/tmp/heudiconvDCMqb920gwc'), PosixPath('/tmp/heudiconvDCM3w5cw9bq')]
E    +  where [PosixPath('/tmp/heudiconvDCM42wzdx58'), PosixPath('/tmp/heudiconvDCMqb920gwc'), PosixPath('/tmp/heudiconvDCM3w5cw9bq')] = list(<generator object Path.glob at 0x7fa918d4df20>)
E    +    where <generator object Path.glob at 0x7fa918d4df20> = <bound method Path.glob of PosixPath('/tmp')>('heudiconv*')
E    +      where <bound method Path.glob of PosixPath('/tmp')> = PosixPath('/tmp').glob
E    +        where PosixPath('/tmp') = Path('/tmp')
E    +          where '/tmp' = <function gettempdir at 0x7fa924f2e040>()
E    +            where <function gettempdir at 0x7fa924f2e040> = tempfile.gettempdir
------------------------------ Captured log call -------------------------------
main.py                    262 INFO     Running heudiconv version 0.9.0 latest 0.9.0
main.py                    262 INFO     Running heudiconv version 0.9.0 latest 0.9.0
main.py                    291 INFO     Need to process 0 study sessions
=============== 1 failed, 55 passed, 4 skipped in 203.21 seconds ===============
The command "coverage run `which py.test` -s -v heudiconv" exited with 1.```

[codecov[bot]]

Codecov Report

Merging #512 (6e95e14) into master (f44c099) will increase coverage by 0.05%. The diff coverage is 89.87%.

:exclamation: Current head 6e95e14 differs from pull request most recent head f78fdef. Consider uploading reports for the commit f78fdef to get more accurate results

@@            Coverage Diff             @@
##           master     #512      +/-   ##
==========================================
+ Coverage   77.28%   77.34%   +0.05%     
==========================================
  Files          41       42       +1     
  Lines        3108     3098      -10     
==========================================
- Hits         2402     2396       -6     
+ Misses        706      702       -4     

Impacted Files	Coverage Δ
heudiconv/utils.py	90.56% <ø> (+0.36%)	:arrow_up:
heudiconv/dicoms.py	82.11% <82.05%> (ø)
heudiconv/convert.py	86.98% <88.88%> (-0.16%)	:arrow_down:
heudiconv/_tmpdirs.py	100.00% <100.00%> (ø)
heudiconv/main.py	77.09% <100.00%> (+0.53%)	:arrow_up:
heudiconv/parser.py	92.63% <100.00%> (-0.08%)	:arrow_down:
heudiconv/tests/test_heuristics.py	100.00% <100.00%> (ø)
heudiconv/tests/test_tarballs.py	100.00% <100.00%> (ø)

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