S3 method issues

[joshuaulrich]

Description

With R-devel (83995-ish) R CMD check and _R_CHECK_S3_METHODS_SHOW_POSSIBLE_ISSUES_=true shows the following:

Package xts:
Mismatches for apparent methods not registered:
str:
  function(object, ...)
str.replot_xts:
  function(x, ...)

time:
  function(x, ...)
time.frequency:
  function(x)

as.POSIXct:
  function(x, tz, ...)
as.POSIXct.tis:
  function(x, offset, tz, ...)

Apparent methods for exported generics not registered:
  as.xts.tis tformat.default

Need to register S3 methods for:

• str.replot_xts()
• as.POSIXct.tis()
• as.xts.tis()
• tformat.default()

And rename time.frequency() to time_frequency() (or delete because it's not used).