Working AD solver for ipeps

Tested with U0 and SU2 for Heisenberg and Kondo-Necklace model.
Pattern does not work with AD at this state.

New framework for handling PEPS

Implementation of PEPS class and implementation of the CTM algorithm.
Compiles an runs fine but no tests so far.

Added support for distributed memory parallelization via MPI by using cyclops tensor library (https://github.com/cyclops-community/ctf)

The interface to plain tensor operations is now extended and also contains plain matrix- and vector operations.
This allows to use arbitrary linear algebra libraries as far as the respective Interface is present.
Supported libraries so far:

1. Eigen + Eigen unsupported tensor module
2. Eigen + ndarray library for tensor manipulations
3. Cyclops library (distributed memory and MPI parallelization)

Other changes:

• Improved cmake build system
• Various bug fixes

Introduced interface to plain tensor operations

New interface class for all plain tensor operations, so that the library for plain tensor operations can be exchanged.
So far, supported libraries are Eigen tensor and array tensor.

Simple MPS layer was added for performance benchmarks.

Basic tensor operations completed

Tensors without symmetry, with arbitrary Abelian symmetry and with SU(2) symmetry are supported with arbitrary rank.
Arbitrary permutations work which in allows arbitrary contractions of the tensors.
gcc and clang (with and without libc++) can be used to build the package.