Safe-Guards preventing segfaults for incorrect inputs (python interface)

See merge request hacc/genericio!5

throw an error for missing variables and multiple-defined
 variables if variables are passed to the reader interface

explicitly define the stride for numpy arrays

Updated Docs, New default for python module

See merge request hacc/genericio!4

allow reading single source rank in py module

add more class methods to py interface, whitespace

non-MPI version for (new) Python interface (only affects new_python)

See merge request hacc/genericio!3

allows suprressing the "Read [...] variables ... " stdout logs

always compile a non-mpi version of the python library, which can be loaded by setting the GENERICIO_NO_MPI env variable

python interface updates and balancing option

See merge request hacc/genericio!2

CMakeLists and new python interface

See merge request hacc/genericio!1