ENH: miniconda

[kaczmarj]

Fixes #147

Breaking changes:

• replaces env_name and preinstalled with create_env and use_env.
  • create_env creates a new environment with conda create.
  • use_env assumes the environment already exists and installs things into it.

Adds ability to create conda environment from yaml file.
Fixes environment variable logic. Environment variables are included when necessary and at most once.

[codecov-io]

Codecov Report

Merging #173 into master will decrease coverage by 0.45%.
The diff coverage is 51.72%.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   82.31%   81.86%   -0.46%     
==========================================
  Files          33       33              
  Lines        1453     1472      +19     
  Branches      140      146       +6     
==========================================
+ Hits         1196     1205       +9     
- Misses        192      198       +6     
- Partials       65       69       +4

Impacted Files	Coverage Δ
neurodocker/tests/test_neurodocker.py	94.11% <ø> (ø)	⬆️
neurodocker/interfaces/tests/test_miniconda.py	100% <ø> (ø)	⬆️
neurodocker/generators/singularity.py	77.96% <100%> (+0.37%)	⬆️
neurodocker/interfaces/interfaces.py	80% <46.15%> (-5.16%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8607690...eb86ff9. Read the comment docs.