ENH: Use EDM Gromacs 2019 Egg

[flongford]

This PR approaches #21 by including the gromacs-2019.4-1 egg from EDM as a dependency. By doing so we also need to update the syntax used by each BaseGromacsCommand subclass to include the gmx executable in each generated command. Additional changes are also made to deal with Gromacs commands that were deprecated between versions 4.6.7 and 2019.4

Gromacs 2019.4 is now assumed as the default build - consequently, the PR also closes #3, #4, and #8

Change log

• EDM core dependency gromacs-2019.4-1 egg included (from enthought/lgpl)
• Bumped Travis CI EDM version to 2.1.0 (including update to RH6 in order to support)
• BaseGromacsCommand class has new executable attribute in order to support gmx command syntax
• New classes Gromacs_solvate and Gromacs_insert_molecules in order to replace deprecated genbox command
• Gromacs_select refactored to incorporate new naming convention
• Gromacs_mdrun refactored to incorporate new syntax for MPI runs

[codecov-io]

Codecov Report

Merging #23 into master will increase coverage by 3.92%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #23      +/-   ##
=========================================
+ Coverage   94.88%   98.8%   +3.92%     
=========================================
  Files          35      55      +20     
  Lines         840    2012    +1172     
  Branches      101     128      +27     
=========================================
+ Hits          797    1988    +1191     
+ Misses         30      10      -20     
- Partials       13      14       +1

Impacted Files	Coverage Δ
...ce_gromacs/core/tests/test_base_gromacs_command.py	100% <100%> (ø)
force_gromacs/core/base_gromacs_command.py	94.82% <100%> (+0.18%)	:arrow_up:
force_gromacs/commands/gromacs_commands.py	100% <100%> (ø)	:arrow_up:
...ce_gromacs/commands/tests/test_gromacs_commands.py	100% <100%> (ø)
force_gromacs/tests/dummy_classes.py	100% <0%> (ø)
...ources/molecule/tests/test_molecule_data_source.py	100% <0%> (ø)
...omacs/data_sources/molecule/tests/test_molecule.py	100% <0%> (ø)
...tion_listeners/hpc_writer/tests/test_hpc_writer.py	100% <0%> (ø)
...line/tests/test_base_gromacs_simulation_builder.py	100% <0%> (ø)
... and 15 more

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