Angles: added function wdstr_nls2()

* Angles: added function wdstr_nls2():
  * returns Dutch two-letter wind-direction abbreviation
  * is a wrapper for pastr_nls()
Converted subroutines lowercase(), uppercase(initial)() to function

* Converted subroutines lowercase(), uppercase(initial)() to function:
  * lowercase(), uppercase() and uppercaseinitial()
  * more flexible, can be used in e.g. write statement
  * no longer changes original string by default
  * replace "call lowercase(str)" with "str = lowercase(str)"