feat/format_upgrade: handle multiple $refs

[ganjigajanan]

Added support for parsing multiple refs (list) instead of single ref. this is due to format changes made & standardized across gen3 dictionaries.

Please dont merge the changes into master yet. it cant not be used until all the gen3 dictionaries are migrated to new format

[frickjack]

• can we add some kind of a unit test that exercises the code with a dictionary that has a list of refs?
• does this change the object level API of the library? So if I have code that currently loads a dictionary and traverses its data structure - is something that was a string yesterday become a list today?

[ganjigajanan]

All the refs properties which generally refers to _terms.yaml file and occasionally other yamls are converted to list instead of string to support multiple definitions a particular property can have. Earlier code travis tests assumed it to a single string and process. The change is to simply convert it to list if not it already is and parse them.

This has no impact on actual node schemas or definitions and already been implemented in GDC.

Refer below pr for more details
NCI-GDC/gdcdictionary@d4758e7

[lgtm-com]

This pull request introduces 1 alert when merging 464fa20 into 71db87f - view on LGTM.com

new alerts:

• 1 for Syntax error

[lgtm-com]

This pull request introduces 1 alert when merging f71c345 into 71db87f - view on LGTM.com

new alerts:

• 1 for Syntax error

[lgtm-com]

This pull request introduces 1 alert when merging cca1449 into 71db87f - view on LGTM.com

new alerts:

• 1 for Syntax error