Mark member functions of coordinate container as const

They qualify for it.
[bugfix] Fix periodic grids for equidistant and tensor product grids

The code is now fully functional!
[new feature] add file sequences.hh with methods for easy coordinate sequence construction

With Yasp being a tensorproduct grid, one needs to construct vectors with monotonous
sequences. This file provides functionality to do so. These include rather easy locally
equidistant methods, but also more involved ones like splitting a range according to
a geometric series.

Work on this file is not completely done yet.