Yu/parallelize - close #83 & changed MGP

[YuuuXie]

There are a few changes:

1. parallelized update_L_alpha function, which modified gp.py and gp_algebra.py
2. The previous MGP has the gp model as its attribute (i.e. self.GP), which is redundant, I eliminated this
3. The previous MGP builds up the spline functions when creating a MappedGaussianProcess object, I changed it to only creating an empty container (i.e. a spline class object without coefficients), and the coefficients can be fitted by calling set_values function. This change is for future's parallelization of MGP prediction

[nw13slx]

why all the MPI command is commented out? maybe we can leave a note in the pull request that this is yet to be tested.

[YuuuXie]

I think we might be able to use multiprocessing.shared_memory instead of mpi4py. I tested it, but there's some conflict inside of ASE's own parallel setting, calling QE and this MGP parallelization.
The tricky part is we need to use mpirun -n xx python **.py if we use mpi4py, I think we must avoid this, and trying multiprocessing is safer.

BTW, the shared memory feature of multiprocessing is new (only supported) in python 3.8....... We might need to transfer to this version....

[nw13slx]

Is NVT available?

[YuuuXie]

It is available. otf_setup.py is not the unit test file, the unit test file is test_otf.py which contains all MD engines. I modified it only because I used it in the documentation (tutorial)