Fix PES figure generation in png format

[xiaoruiDong]

Motivation or Problem

One of the arguments of rmgpy.pdep.draw is renamed from format to file_format in a previous commit, but the corresponding codes are not fully refactorized, causing failures in generating PES figures in png format. This will further hinder the PES from being generated and displayed on the RMG-website.

Description of Changes

rename format to file_format .

Testing

By calling the draw function with file_format="png", it can generate PES figure in .png format

[alongd]

Thanks!!
Could you also fix the docstring on line 241 (format to file_format as well)?

[codecov]

Codecov Report

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The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #1802   +/-   ##
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  Coverage          ?   42.74%           
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  Files             ?       81           
  Lines             ?    21078           
  Branches          ?     5491           
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  Hits              ?     9010           
  Misses            ?    11054           
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Impacted Files	Coverage Δ
rmgpy/pdep/draw.py	4.33% <0%> (ø)

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[xiaoruiDong]

Thanks @alongd. Docstring changed as well.

[alongd]

Thanks!