Forte export

[zhanyuanucb]

This PR fixes #688

Description of changes

Added export method to Pipeline that checks a predefined environment variable FORTE_EXPORT_PATH and export pipeline config into it.

Possible influences of this PR.

Describe what are the possible side-effects of the code change.

Test Conducted

Added tests in tests/forte/pipeline_test.py

• export should do nothing if FORTE_EXPORT_PATH is not defined
• export should raise FileNotFoundError if FORTE_EXPORT_PATH is assigned but does not exists
• export should dump pipeline config to FORTE_EXPORT_PATH if it exists
• users should be able to export a pipeline multiple times, with different export names

[codecov]

Codecov Report

Merging #690 (0e26397) into master (343e8c0) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #690      +/-   ##
==========================================
+ Coverage   80.86%   80.94%   +0.07%     
==========================================
  Files         242      242              
  Lines       17809    17883      +74     
==========================================
+ Hits        14401    14475      +74     
  Misses       3408     3408              

Impacted Files	Coverage Δ
forte/pipeline.py	93.93% <100.00%> (+0.21%)	⬆️
tests/forte/pipeline_test.py	97.22% <100.00%> (+0.20%)	⬆️

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[hunterhector]

Looks very nice, thank you!