Feature: get distribution from node

[Roman223]

Now user can take a conditional distribution from node.
Tests are included.

[codecov-commenter]

Codecov Report

Merging #76 (a25b4ef) into master (4f7fe72) will increase coverage by 1.22%.
The diff coverage is 80.48%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   64.35%   65.57%   +1.22%     
==========================================
  Files          32       32              
  Lines        2250     2298      +48     
==========================================
+ Hits         1448     1507      +59     
+ Misses        802      791      -11     

Files Changed	Coverage Δ
bamt/nodes/base.py	69.76% <66.66%> (-0.24%)	⬇️
bamt/nodes/mixture_gaussian_node.py	78.87% <70.00%> (+1.26%)	⬆️
bamt/nodes/conditional_mixture_gaussian_node.py	87.03% <72.72%> (+2.57%)	⬆️
bamt/networks/base.py	52.79% <80.00%> (+4.16%)	⬆️
bamt/nodes/conditional_gaussian_node.py	86.23% <80.00%> (+0.66%)	⬆️
bamt/nodes/logit_node.py	86.15% <83.33%> (+1.15%)	⬆️
bamt/nodes/gaussian_node.py	85.07% <85.71%> (+0.69%)	⬆️
bamt/nodes/discrete_node.py	98.41% <88.88%> (-1.59%)	⬇️
bamt/nodes/conditional_logit_node.py	79.43% <90.00%> (+0.86%)	⬆️