Refactor lduLduBase

[greole]

The current structure of the lduLduBase class is not ideal and several changes should be considered:

1. rename the class to make the name more descriptive, a candidate would be SegregatetedCoupledSolverBase
2. Instead of deriving from HostMatrix and IOGKOMatrixHandler an intermediate class LduCsrWrapper should be implemented

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The new SegregatetedCoupledSolverBase class has the following responsibilities

Data members:

• vector of DevicePersistent initial guesses and rhs

Methods:

• generate_solver
• generate_preconditioner
• initialisation of initial guesses and rhs
• solve_impl

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The new intermediate LduCsrWrapper class has the following responsibilities:

Data members:

• DevicePersistent sparsity pattern
• DevicePersistent LduCsrMapping
• DevicePersistent values

Methods:

• init/update sparsity pattern
• init/update lduCsrMapping
• init/update values

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Most of the functionality could be implement as a mixin which provide the needed update and init functions.

This depends on the implementation of DevicePersistent class #49