Improved checkARstar

More flexible: multiple high frequencies allowed. More concise: `isARstar` is not returned anymore, `shortSeq` is shorten. Slightly faster: `sweep` in `all_coef` replaced with `mat %*% diag(vec)`.
Fixed conversions among lag structures

In case of not full lag structure `fmls` is changed to `mls`, `dmls` prompts an error. `shortSeq` is used to obtain a short call of not full lag vector, e.g. `shortSeq(c(1,2,3,5,7))` returns a call `c(1:3,5,7)`.
Merge pull request #13 from Julius-V/patch-4

Improved checkARstar