fix MML custom LJ properties conversion test

[#41834627]

Before fix: epsilon and sigma were only set correctly in first 
pairwise LJ property object generated.

Expected second element:

  { 
    "element1": 0, 
    "element2": 1, 
    "sigma": 0.38754093170166015, 
    "epsilon": -0.3464285731315613 
  } 

Actual second element:

  { 
    "element1": 0, 
    "element2": 1, 
    "sigma": 0, 
    "epsilon": 0 
  } 
Makefile: convert-mml target
Guardfile: large speed improvements working on tests

The Guardfile is updated so to optimize what is run when
just making changes to tests themselves or mml parser or
mml conversion scripts.

Specifically:
- only specific vows or mocha test is run when that test is updated
- when mml-parser is changed first just conversion tests are run,
if these tests pass then only conversion is run
- when mw-batch-convertor or post-batch-processor.rb are changed
just conversion is run