Replaces prints with logging module.

[rutgerfick]

In preparation of pypi we change all prints to appropriate logging messages.

closes #82

[coveralls]

Pull Request Test Coverage Report for Build 593

• 43 of 60 (71.67%) changed or added relevant lines in 6 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.04%) to 82.559%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dmipy/data/saved_data.py	3	4	75.0%
dmipy/optimizers_fod/csd_cvxpy.py	1	3	33.33%
dmipy/tissue_response/white_matter_response.py	6	9	66.67%
dmipy/distributions/distribute_models.py	1	5	20.0%
dmipy/core/modeling_framework.py	29	36	80.56%
<!--	Total:	43	60

Files with Coverage Reduction	New Missed Lines	%
dmipy/tissue_response/white_matter_response.py	1	76.6%
dmipy/distributions/distribute_models.py	2	72.12%
<!--	Total:	3

Totals
Change from base Build 591:	-0.04%
Covered Lines:	14122
Relevant Lines:	16650

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #83 into master will increase coverage by 0.04%. The diff coverage is 71.66%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
+ Coverage    81.7%   81.74%   +0.04%     
==========================================
  Files          64       64              
  Lines        5543     5550       +7     
  Branches      655      655              
==========================================
+ Hits         4529     4537       +8     
  Misses        840      840              
+ Partials      174      173       -1

Impacted Files	Coverage Δ
dmipy/optimizers/brute2fine.py	83.6% <100%> (+0.09%)	:arrow_up:
dmipy/distributions/distribute_models.py	69.08% <20%> (-0.27%)	:arrow_down:
dmipy/optimizers_fod/csd_cvxpy.py	89.13% <33.33%> (+0.11%)	:arrow_up:
dmipy/tissue_response/white_matter_response.py	73.68% <66.66%> (+1.53%)	:arrow_up:
dmipy/data/saved_data.py	68.81% <75%> (+0.16%)	:arrow_up:
dmipy/core/modeling_framework.py	71.16% <80.55%> (+0.01%)	:arrow_up:
dmipy/core/acquisition_scheme.py	78.9% <0%> (+0.47%)	:arrow_up:

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