Implementes mav tails for smooth_age_5 and smooth_age_5_mav

[cimentadaj]

Here is a summary of the changes:

• Apart from the good old mav, we now have mav_tails. This should return the same as mav except for the tails where cascading is applied automatically. mav_tails is in the same help page as mav and this is documented and exported, as mav.

• smooth_age_5_mav gains a new argument called tails which is set to FALSE to avoid breaking stuff backwards. When set to TRUE, we use mav_tails otherwise, mav. This is documented in smooth_age_5_mav docs.

• smooth_age_5 now allows to fill in the tails with arg 'Cascade'. This just calls smooth_age_5_mav with tails = TRUE. All documentation has now integrated this option.

• Added additional tests for smooth_age_5_mav

All checks are passing locally.

[cimentadaj]

If you need me to change anything (for ex, you don't want a tails arg on smooth_age_5_mav), just let me know and I'll commit extra changes.

[cimentadaj]

mavNew has an extra line for replacing the last value when OAG is TRUE with the original value (aside from the previos fill_mav). However, your points are spot on. This is the new setup:

*mav gains a tails arg which is set to FALSE by default. This simply calls mav_tails if set to TRUE. mav_tails is currently not exported because it just seemed redundant since it needs the result of mav as an argument. Users can now just mav(..., tails = TRUE). All docs have been updated with these changes.

• smooth_age_5 and smooth_age_5_mav now call mav and pass along the tails arg.

All checks pass locally and all docs have been updated.