CRAN release 0.23-4

Updated the DEMA() function to avoid multiple calls to EMA(). This required multiple changes to the EMA() code, both in R and C. This also required us to add try.xts() and reclass() to DEMA(), since EMA() had been handling it.We also added other checks from EMA() (the value of n and the number of input columns).

Moved NA handling (leading and non-leading) from R to C. This makes it easier to call the ema() C function from other C functions. That avoids the overhead in R function calls. Also moved the n and ratio calculations from R to C. This also makes it easier to call the ema() C function.

Changed how xts' naCheck() C function was called. Now it's consistent with how xts calls the zoo_lag() and zoo_coredata() C functions.

This release also includes some minor housekeeping for CRAN: Updated some URLs to use https to avoid redirects. And examples that need internet connections are only run in interactive sessions, in order to avoid long R CMD check times.