Addition of new shortcuts in specification file

[jimagaffney]

Adding three new shortcuts to the specification definition:

$(MERLIN_SAMPLE_VECTOR) is substituted for "$(SAMPLE_COLUMN_1) $(SAMPLE_COLUMN_2) ..." $(MERLIN_SAMPLE_NAMES) is substituted for "SAMPLE_COLUMN_1 SAMPLE_COLUMN_2 ..." $(MERLIN_SPEC) is substituted for the path to the *.expanded.yaml file in the merlin_info directory

Changelog:

• In the special_vars method of the MerlinStudy class, add reference and definitions of the new shortcuts
• Added new method get_sample_labels to MerlinStudy class. This generalizes the existing sample_labels property to allow the labels to be read from a user-specified MerlinSpec instance. This is needed to avoid access to the expanded_spec property before special_vars are defined, which causes an AttributeError
• Update to existing sample_labels property to be consistent with new get_sample_labels method

Tests Performed:

• Successful generation and run of "hello world" example script
• Successful generate and run of existing distributed workflow modified to use new shortcuts

[lucpeterson]

• [ ] I'd like to change the spec shortcut name (and add a few others). What about these names?

• $(MERLIN_SPEC_RAW) --> *.orig.yaml

• $(MERLIN_SPEC_RUN) --> *.partial.yaml

• $(MERLIN_SPEC_EXPANDED) --> *.expanded.yaml

• [ ] We should also add these to the documentation

• [ ] Lint check is failing

• [ ] Update the CHANGELOG

[ryannova]

Resolved by merge request #375