Move user editable headers back to src

Although these files are not declared in the FILES file in each src
directory, they are needed because they contain user code.
They should not be declared in FILES because their existance is
declared within the CDL file.

Removal of two somehow forgotten lxx files within src was needed:
AIS_Drawer.lxx and math_FunctionSetRoot.lxx.
Merge pull request #398 from tpaviot/review/move-header-files-into-src-take3

Move user editable headers back to src