Blurb::
Specify the number of basis functions to be used in the random field representation.
THIS IS AN EXPERIMENTAL CAPABILITY.
Description::
This control allows the user to specify the number of basis functions to be
used in the random field representation (e.g. either a Karhunen-Loeve or Principal Components expansion).
Typically, a small number of basis functions (3-5) will be sufficient to represent a significant
amount of the variance in the response field.  However, this depends on the particulars of the problem.
THIS IS AN EXPERIMENTAL CAPABILITY UNDER ACTIVE DEVELOPMENT.

*Default Behavior*
If the user specifies neither ``trunction_tolerance`` nor ``expansion_bases``, then
the number of expansion bases that captures 95% of the variance will be used.
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