Change r/ and c/ as a way to clear the fields to r/nil and c/nil

[JuliaPoo]

Fixes #149, Fixes #185.

[Fidget-Spinner]

What happens when you pass this as a role to another command? e.g. the charge command?

[JuliaPoo]

It errors out with invalid role format, since I added the check within the Role.isValidRoleName thingy

[Fidget-Spinner]

Ok needs merge conflict fixing/

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 66.06%. Comparing base (8f4e2ad) to head (8a4effe).

Files	Patch %	Lines
.../seedu/address/logic/parser/EditCommandParser.java	50.00%	0 Missing and 1 partial ⚠️
src/main/java/seedu/address/model/cca/Cca.java	0.00%	0 Missing and 1 partial ⚠️
src/main/java/seedu/address/model/roles/Role.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #186      +/-   ##
============================================
- Coverage     66.15%   66.06%   -0.10%     
- Complexity      553      555       +2     
============================================
  Files            95       95              
  Lines          2163     2163              
  Branches        219      219              
============================================
- Hits           1431     1429       -2     
  Misses          652      652              
- Partials         80       82       +2     

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[JuliaPoo]

Okay I did the merge.