Inexact HMatrix

[maltezfaria]

@PierreMarchand20 and @DuduGuima: this draft PR is a place where we can discuss ideas and code to implement the a lazy coarsened HMatrix type (and accompanying functionality). A rough outline (feel free to edit it):

• Augment RkMatrix so that you can store the relative tolerance
• Modify the partial aca algorithm to push the relative tolerance to the RkMatrix constructor
• Create a type where we can store an HMatrix as well as a desired tolerance for the product
• Overload that type so that we only use the minimum rank of each low-rank block needed for the desired tolerance in the matrix-vector product

[codecov]

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