Finer-grain customization of LaTeX output

[Lelio-Brun]

This PR makes 3 (conservative) contributions to the LaTeX output:

1. \ottfirstprodnewline allows customizing the linebreak occurring on the very first line after the ::=
   For example with \renewcommand{\ottfirstprodnewline}{} in combination with an accordingly redefined \ottfirstprodline we can obtain

   x ::= A
       | B
       | C
   

   instead of

    x ::= 
        | A
        | B
        | C
   

2. \ottlastpremise allows customizing the premise separator after the last premise. This is useful if, for example, one wants to use mathpartir.
3. \ottdrulename<rule_name> allows typesetting the name of a rule. It can be superior to \ottdrulename{<rule_name>} because the former will result in a LaTeX error if the rule name changes in the Ott file.