Refactor: clean up BLACS initialization in Parallel_2D

[jinzx10]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

What's changed?

Code relevant to the initialization of BLACS grid & its information is made slightly easier to read & maintain.

This PR is just a stage-1; further refactoring towards more convenient manipulation on 2d-block-cyclic data structure will follow.

Question

I gather BLACS would split the given communicator to get some new ones for its own purpose when initializing the process grid, so it might be unnecessary to create a Cartesian MPI communicator in advance, which means we might get rid of the function mpi_create_cart(MPI_Comm). However, I'm not completely sure whether a Cartesian MPI grid would have performance benefit; this needs to be verified by an MPI/ScaLAPACK expert.

Any changes of core modules? (ignore if not applicable)

No.

[mohanchen]

Nice job!