[Wishlist from Aachen] control of the L() ordering

[levandov]

Intro:The use of L in the ordering definition works in the explicit ring definition. Sometimes (for big problems) this is the only way to get the result, especially in the Singular:Letterplace setting.
Problem: many constructions in the procedures are now done with the help of ringlist.
Wish:It is very desirable to have the possibility to use ringlist in order to set the ordering with the L block. Also other mechanisms might be possible. At the moment the only way to set the L ordering in the library is to work with strings.