Blurb::
An accuracy tolerance that is used to guide rounding during rank adaptation.
Description::
The rounding tolerance is used to measure how closely one can approximate
a functional tensor train representation at a step of the optimization
procedure using one of lower ranks. If this tolerance is small, then less
compression is achieved but greater accuracy is obtained. If this
tolerance is big, greater compression is achieved at the expense of
accuracy.

*Default Behavior*

If this tolerance is not specified, then we adopt the internal C3 default.
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