Force a Buffer * argument to MathData constructor

Force a Buffer * argument to MathData constructor

In order to ensure that MathData objects have a valid buffer, the
default MathData() constructor is deleted. This means that a buffer
shall be specified for each MathData object created.

This is fairly mechanical, actually. In particular, in most
InsetMathXxx cases, in MathData and in MathParser, the available
buffer_ member is used.

More specific cases:
- lyxfind.cpp takes the buffer from the Cursor

- calls to vector<MathData>::resize take an additional
  MathData(buffer_) parameter. There are cases where resize actually
  remove cells, and in this case clear() or even erase() have been
  used.

- in InsetMathMacroTemplate, the optional parameters of the
  constructors cannot be allowed anymore (a default value cannot
  depend on another parameter). Therefore there a now two constructors
  instead.

- in MathAutoCorrect.cpp, the MathData objects are not bound to a
  buffer, so that std::nullptr is used instead.

- in MathExtern, use a buffer when one is specified, std::nulptr
  instead.