Add missing basic gremlin traversal functions

[kevmojay]

Gremlex is in early stages of development and thus missing a ton of functions. We want your help to add them for pretty easy PR points for your t-shirt from hacktoberfest. Below is all the simple PRs you can make.

Take a look at the additional references section in each link to see the possible ways to use the functions. We don't currently support functions that take a Traversal, Consumer, and Scope (there may be others. Basically anything that turns into a string, int, boolean, or double is fine)

An example of what many of these PRs will be like is https://github.com/Revmaker/gremlex/pull/23/files

• [x] aggregate-step (easy)
• [ ] as-step (medium)
• [x] barrier-step (easy)
• [x] cap-step (medium)
• [x] coin-step (easy)
• [x] constant-step (easy)
• [x] count-step (easy)
• [x] cyclicPath-step (easy)
• [ ] from-step (easy)
• [x] group-step (easy)
• [x] groupCount-step (easy)
• [x] hasId from has-step (medium)
• [x] hasKey from has-step (medium)
• [x] hasNot from has-step (easy)
• [x] id-step (easy)
• [x] identity-step (easy)
• [x] inject-step (easy)
• [ ] is-step (easy)
• [x] key-step (easy)
• [ ] label-step (easy)
• [ ] limit-step (long only) (easy)
• [ ] loops-step (easy)
• [ ] math-step (easy)
• [x] max-step (easy)
• [ ] mean-step (easy)
• [x] min-step (easy)
• [ ] order-step (easy)
• [ ] path-step (easy)
• [ ] peerPresure-step (easy)
• [ ] profile-step (easy)
• [ ] project-step (medium)
• [ ] property-map (medium)
• [ ] range-step (easy)
• [ ] sample-step (easy)
• [ ] select-step (support for string and multiple strings) (medium)
• [ ] simplepath-step (easy)
• [ ] skip-step (easy)
• [x] store-step (easy)
• [x] sum-step (easy)
• [x] tail-step (easy)
• [ ] taillimit-step (easy)
• [ ] unfold-step (easy)

[JohannesSoots]

Made pull requests for id-step and identity-step. #48 #49

[luisfmcalado]

Inject and tail function pull requests. ❤️ #67 #68