MAINT: change comparisons to None
MAINT: not equal comparison to None
add algopy.logdet, remove algopy.det and add algopy.prod to the list of elementary functions (i.e., it is not a compound function anymore, aka has it's own node in the computational graph)
commented logdet implementation
add UTPM.lu
add keyword k to algopy.tril(A,k=0) and algopy.triu(A,k=0)
add UTPM.pb_lu
(re-) add algopy.det and algopy.logdet
bugfix in logdet