Missing bits of the std::string -> hullType change of InsetMathHull::type_.

Missing bits of the std::string -> hullType change of InsetMathHull::type_.
Now we write again \begin{align} instead of \begin{4} to .lyx and .tex.
	* src/mathed/InsetMathHull.C
	(InsetMathHull::header_write): write type name, not type_.
	Use switch instead of if for better readability.
	(InsetMathHull::footer_write): ditto
	(InsetMathHull::mutate): write type name, not type_
	(InsetMathHull::normalize): ditto
	(InsetMathHull::infoize): ditto

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15204 a592a061-630c-0410-9148-cb99ea01b6c8