Update the way we import external libraries by using only the necessary modules

[skoudoro]

This PR is the beginning of #457.

The goal is to make our scripts load faster by importing only the functions/modules we need from VTK.

[codecov]

Codecov Report

Merging #470 (c3c3386) into master (48939ae) will increase coverage by 0.17%.
The diff coverage is 92.89%.

@@            Coverage Diff             @@
##           master     #470      +/-   ##
==========================================
+ Coverage   89.03%   89.21%   +0.17%     
==========================================
  Files          33       34       +1     
  Lines        7112     7230     +118     
  Branches      836      836              
==========================================
+ Hits         6332     6450     +118     
  Misses        547      547              
  Partials      233      233              

Impacted Files	Coverage Δ
fury/__init__.py	65.71% <20.00%> (ø)
fury/io.py	81.01% <72.72%> (-0.12%)	⬇️
fury/window.py	81.90% <77.77%> (-0.05%)	⬇️
fury/utils.py	85.82% <84.00%> (-0.04%)	⬇️
fury/actors/peak.py	89.94% <90.00%> (-0.06%)	⬇️
fury/actor.py	89.62% <90.21%> (-0.02%)	⬇️
fury/actors/odf_slicer.py	87.61% <100.00%> (ø)
fury/actors/tests/test_peak.py	97.95% <100.00%> (ø)
fury/colormap.py	93.89% <100.00%> (ø)
fury/convert.py	80.00% <100.00%> (ø)
... and 11 more

[skoudoro]

This PR is ready to go. Can you have a look @Garyfallidis and @guaje?

Thank you

[Garyfallidis]

@skoudoro there is currently a conflict.
I wonder if we should have one file that does all the vtk imports and then just load functions from that file.
For example,

from fury.fvtk import vtkPolyDataMapper

[Garyfallidis]

And then in fvtk.py we add the

import vtkmodules.vtkCommonCore as ccvtk
Polydata = ccvtk.vtkPolyDataMapper
or
vtkPolydata = ccvtk.vtkPolyDataMapper

In this way in the other scripts you can just do

from fury.fvtk import vtkPolyData
or 
from fury.fvtk import Polydata

Also what speedup do you see?

[Garyfallidis]

This is great and much needed!
Next steps are:

a) update molecular.py file to use new gig.
b) update all tutorials and demos.

Thank you @skoudoro !

[Garyfallidis]

@skoudoro on your upcoming PR please also report timings. How much faster is importing in this new way than before?
For example, how much time an application takes to load now and before ?