Refactor: wrappers -> modules

[skim0119]

Resolve #168

[armantekinalp]

LGTM

[bhosale2]

LGTM mostly, super minor change.

[codecov-commenter]

Codecov Report

Merging #177 (d693872) into update-0.3.0 (f9b891f) will decrease coverage by 0.06%.
The diff coverage is 0.00%.

@@               Coverage Diff                @@
##           update-0.3.0     #177      +/-   ##
================================================
- Coverage         87.60%   87.53%   -0.07%     
================================================
  Files                43       44       +1     
  Lines              2815     2817       +2     
  Branches            367      367              
================================================
  Hits               2466     2466              
- Misses              328      330       +2     
  Partials             21       21              

Impacted Files	Coverage Δ
elastica/boundary_conditions.py	95.00% <ø> (ø)
elastica/modules/__init__.py	100.00% <ø> (ø)
elastica/modules/base_system.py	98.41% <ø> (ø)
elastica/modules/callbacks.py	94.87% <ø> (ø)
elastica/modules/connections.py	93.44% <ø> (ø)
elastica/modules/constraints.py	100.00% <ø> (ø)
elastica/modules/damping.py	94.87% <ø> (ø)
elastica/modules/forcing.py	100.00% <ø> (ø)
elastica/modules/memory_block.py	100.00% <ø> (ø)
elastica/systems/__init__.py	87.50% <ø> (ø)
... and 1 more

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[bhosale2]

LGTM.