Update of sandbox doc

In the documentation to sandbox.rst, I eliminated the references to 
- tsa.movmean
- tsa.movmoment
- tsa.movorder
- tsa.movvar
and instead included a comment and a link to the corresponding Pandas-functions.
Left "tsa.movstat", as I am unsure if it is covered by Pandas.
Also, this is my first commit, so I wanted to do something small.