Refactor of GromacsPipeline

[flongford]

This PR refactors the GromacsPipeline class, in particular:

• Converts the named_steps property on GromacsPipeline into a traits Property class, rather than using the @property decorator.
• The getter method for named_steps is also simplified to remove redundant code
• Converts the _iter method into a regular python __iter__ method, that no longer returns the index of iteration.
• Subsequent updates to bash_script and run methods to account for these changes
• Unit testing has also been refactored to reflect these changes and to remove code that was testing functionalities covered by the traits library.

[codecov-io]

Codecov Report

Merging #13 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   94.55%   94.55%           
=======================================
  Files          34       34           
  Lines         826      826           
  Branches       98       98           
=======================================
  Hits          781      781           
  Misses         31       31           
  Partials       14       14

Impacted Files	Coverage Δ
force_gromacs/pipeline/gromacs_pipeline.py	100% <100%> (ø)	:arrow_up:

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