rename `rmo_ex_arnold` to `rexmo_markovian`

[hsloot]

Description

Rename methods to make the package more extendible.

Checklist

• [x] R CMD CHECK successful
• [x] tests included
• [x] documentation included or updated
• [x] commit messages follow commit guidelines

Optional, but recommended:

• [x] code passes lintr::lint_package() without errors

[codecov[bot]]

Codecov Report

Merging #73 (4cdde93) into master (4b85d3d) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #73   +/-   ##
=======================================
  Coverage   96.83%   96.83%           
=======================================
  Files          30       30           
  Lines        1106     1106           
=======================================
  Hits         1071     1071           
  Misses         35       35           

Impacted Files	Coverage Δ
R/sample.R	100.00% <100.00%> (ø)
src/sample.cpp	100.00% <100.00%> (ø)

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