=> operator for more intuitive passing of parameters to R functions

Proposed Changes

Following discussion in #183, adds the proposed => operator in a RProvider.Operators module. The => operator can be used instead of namedParams to define a parameter list to pass to an R function:

R.plot([
    "x" => widgets
    "type" => "o"
    "col" => "blue"
    "ylim" => [0; 25] ])

R.plot [ "x" => Xs; "y" => Ys ]

... instead of:

R.plot(
    namedParams [   
        "x", box widgets; 
        "type", box "o"; 
        "col", box "blue";
        "ylim", box [0; 25] ])

R.plot (namedParams [ "x", box Xs; "y", box Ys ])

I added an additional overload to R functions that takes a list of string obj rather than a dictionary of string obj to enable use of the => operator. The dictionary option is still available so that the change is not breaking.

Documentation has been updated to use the Operators module and =>.

Types of changes

What types of changes does your code introduce to RProvider? Put an x in the boxes that apply

[ ] Bugfix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

[x] Build and tests pass locally
[x] I have added tests that prove my fix is effective or that my feature works (if appropriate)
[x] I have added necessary documentation (if appropriate)

fslaborg / RProvider