Increase date documentation, coverage and functionality (#434)

* Improve date documentation * Update coverage for YearQuarter * Add addMonths/Quarters/Years

Author: jmh530

dub.sdl

@@ -12,7 +12,7 @@ dependency "mir-core" version=">=1.1.106"
 buildType "unittest" {
  buildOptions "unittests" "debugMode" "debugInfo"
  versions "mir_bignum_test" "mir_bignum_test_llv"
- // versions "mir_ndslice_test"
+ // versions "mir_ndslice_test"
  versions "mir_test"
  dflags "-lowmem"
 }