f2py: pass array/scalar directly instead of writing/reading text file

[yunjunz]

It seems straightforward to pass array and scalar variables from Fortran subroutine to Python function, as shown here: https://notmatthancock.github.io/2017/02/10/calling-fortran-from-python.html. Thus, we should eliminate the writing/reading of solid.txt file and pass solid.for results directly to point/grid.py.

Relevant link: https://numpy.org/doc/stable/f2py/f2py.getting-started.html#the-quick-and-smart-way.

Contributions are welcomed!

[yunjunz]

Another solution is to add a C++ layer and compiled by pybind11, to connect Fortran, C++ and Python. There are plenty of examples in isce2 for this, as pointed out by @rtburns-jpl.