Draft: Add logAbsDeterminant for LLT

Mainak Royrequested to merge
Newtech66/eigen:llt_logdet into master

Reference issue

Related to #757, but doesn't solve that specific issue.

What does this implement/fix?

Adds a logAbsDeterminant method for LLT. Could expand the scope to add other determinant methods for LLT, as well as LDLT.

Additional information

I'm very new to this project and I have no idea what I'm doing.

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