[DEV] added a public method to access linear system vectors (instead of direct member access, but this is work in progress in muphys side)
[ENH] dense matrix reallocation now only happens if number of entries is different, even if i1!=i2 or i2!=j2 (this is useful for muphys::TimeAccurate because it reassigns b and static rhs every time step, before it was only not reallocating because it was swapping contents, while efficient this was hardcoding the two plugins together).
[DEV] added a method summing the rows of the different matrix variations (not fully tested).