Introducing tiny matrices

New classes:

• cppmat::matrix2<T> and cppmat::vector<T> for 2-d and 1-d matrices/arrays. Compared to cppmat::matrix the indices are simplified in the sense that periodicity is not allowed. In the future more efficiency updates may be made in these classes.

• cppmat::tiny::matrix2<T,M,N> and cppmat::tiny::vector<T,N> for small 2-d and 1-d matrices/arrays. These classes allocate the data directly (no constructor needed), on the 'stack'. This is considerably faster, but only useful for small arrays.

• All new classes are provided with pybind11 bindings.