PETSc: provide an initializer for matrices that do not require to set sizes, for the sake of LoadBinary

We provide an API for loading matrices from binary file, but so far we needed to call first a method to initialize properly the layout, such as InitSequentialMatrix().

However, these methods require to specify the expected number of rows and columns... and one of Slepc hands on underlines we do not actually need to specify these informations.

MoReFEM should therefore provide as well the API to support a similar case... obviously without lowering the security so we need to be careful and prevent operations that would need the size which would not be correctly set.