[cleanup] Remove base_array and base_array_window.

These classes are internal to ISTL and used nowhere in ISTL.

Note: base_array_unmanaged is used internally by BlockVector, Matrix, and VariableBlockVector and is thus not removed.

Intent-to-merge: 2018-05-23.

changed the description

@markus.blatt Are there reasons against the removal?

Possible impact on !62 and #22

merged

mentioned in commit a7811f02

nope, not that I know of.