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

• aggregate-step (easy)
• as-step (medium)
• barrier-step (easy)
• cap-step (medium)
• coin-step (easy)
• constant-step (easy)
• count-step (easy)
• cyclicPath-step (easy)
• from-step (easy)
• group-step (easy)
• groupCount-step (easy)
• hasId from has-step (medium)
• hasKey from has-step (medium)
• hasNot from has-step (easy)
• id-step (easy)
• identity-step (easy)
• inject-step (easy)
• is-step (easy)
• key-step (easy)
• label-step (easy)
• limit-step (long only) (easy)
• loops-step (easy)
• math-step (easy)
• max-step (easy)
• mean-step (easy)
• 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)
• store-step (easy)
• sum-step (easy)
• 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