Further ASAD refactoring

[joewallwork]

Transfer of ukca:#325 on MOSRS.

Following the completion of ukca:#302, I spotted a few more things in ASAD that could do with refactoring. These include:

• Drop unused arrays and arguments from previous refactoring work.
• Use the more accurate array name permuted_nonzero_map rather than nonzero_map_unordered.
• Details on LU factorisation.
• Clarity of comments.