Release v0.7.0

• Added an implementation of the StateVector concept for some Eigen 3 types (vectors, vector segments,
  column-like matrix blocks and one-dimensional Eigen::Map views). The corresponding header file is loperator/state_vector_eigen3.hpp.
• New classes n_fermion_sector_view and n_fermion_multisector_view that implement N-fermion (multi)sector views of a state vector. The classes are supplemented with free utility functions make_nf(m)s_view(), make_const_nf(m)s_view(), n_fermion_(multi)sector_size() and n_fermion_(multi)sector_basis_states().
• New member typedef hilbert_space::index_types.
• New method hilbert_space::has_algebra().
• New method space_partition::find_connections().
• Renamed the constexpr integer LIBCOMMUTE_MIN_USER_DEFINED_ALGEBRA_ID to min_user_defined_algebra_id so that it does not appear to be a macro.
• In the 3-argument constructor of basis_mapper, change the type of the last argument N from int to unsigned int.
• Removed methods basis_mapper::make_(const_)view_no_ref(). basis_mapper::make_(const_)view() will now return a non-reference view when supplied with a non lvalue-reference argument.
• New CMake option STATIC_ANALYSIS. When enabled, the clang-tidy and cppcheck static analysis tools will be run on the C++ sources of unit tests and examples as part of build process.
• Minor bugfixes in unit tests and improvements in coding style.
• Reformatted C++ sources using clang-format and a style based on LLVM.