New features for eye count and string recognition

eye 1.2.0

Summary of changes:

• eyes() - now returns lists (of class "eyes") for easier access of count data
• set_eye_strings() - set string codes globally! This makes it easier for people using different languages to use eye.
• recodeye: renamed "eyecodes" argument to "eyestrings"
• Change of terminology to "subjects/id" rather than "patients"
• simplified code

Details:

• eyes(): "details" argument creates object of class "eyes_details", allowing access to in depth count statistics and subject ids
• This includes new print methods for class "eyes" and "eyes_details"
• change id and eye arguments to "id_col" and "eye_col" - previous code should not break because of partial argument name matching. However, the new names make it clearer as to what those arguments are for.
• calling set_eye_strings will super-assign the new codes to the internal
  list object eye_codes
• getElem_eyecol now gives precedence to columns that are called "eye" or "eyes" (#30)
  Simplifications:
• simplified pivoting in myop using the pivot_longer regex feature
• removed ...chr arguments from getElem... functions (redundant)
• getElem... now only for vectors (because they were anyways only used for vectors)