Performance Improvement of SPE function

The performance of SPE function was bad because the mvdct function of dtt package is not efficient.

Now, SPE function uses DCT2D function and IDCT2D function instead of mvdct function.

The performance of SPE function was improved.

You can confirm it by the code below.

library(devtools)
install_github("ShotaOchi/imagerExtra")
library(imagerExtra)
g <- grayscale(boats)
SPE(g, 0.1) %>% system.time()

   user  system elapsed 
      0       0       0