Comparison between filebacked and memoryBased pixels

[abuts]

This ticket is part of Re #1147 wich sorted main issues with equal_to_tol but left small independant things unfinished.

By default, filebacked pixels are compared between each others page by page.
Memory based pixels are compared as a whole.

To be able to compare memory based and filebacked pixels one needs to select filebacked pixels page equal to number of pixels located in memory. Currently this can be done manually only.

The purpose of this ticket is to modify equal_to_tol to automate this behaviour and be able to compare filebacked and memory based pixels automatically.

Unit tests test_sqw_pajeOpMethods/test_join:test_join_creates_tmp_filebacked_on_conditions and
test_sqw_pajeOpMethods/test_join:test_join_works_with_file_list_with_nomex_pages illustrates this behavour and should be fixed to work with arbitray pages size. The tests are marked as belonging to #1147 ticket.