Add JL_UMFPACK_PIVOT_TOLERANCE to umfpack.jl (#447)

Looks like this was just omitted for some reason

See UMFPACK manual:

Control [UMFPACK_PIVOT_TOLERANCE]: relative pivot tolerance for threshold partial pivoting with row interchanges. In any given
column, an entry is numerically acceptable if its absolute value is
greater than or equal to Control [UMFPACK_PIVOT_TOLERANCE] times the largest absolute value in the column. A value of 1.0 gives true partial pivoting. If less than or equal to zero, then any nonzero entry is numerically acceptable as a pivot. Default: 0.1.

Smaller values tend to lead to sparser LU factors, but the solution
48 to the linear system can become inaccurate. Larger values can lead to a more accurate solution (but not always), and usually an
increase in the total work.

src/solvers/umfpack.jl

@@ -47,6 +47,7 @@ import ..LibSuiteSparse:
     UMFPACK_PRL,
     UMFPACK_DENSE_ROW,
     UMFPACK_DENSE_COL,
+    UMFPACK_PIVOT_TOLERANCE,
     UMFPACK_BLOCK_SIZE,
     UMFPACK_ORDERING,
     UMFPACK_FIXQ,
@@ -81,6 +82,7 @@ import ..LibSuiteSparse:
 const JL_UMFPACK_PRL = UMFPACK_PRL + 1
 const JL_UMFPACK_DENSE_ROW = UMFPACK_DENSE_ROW + 1
 const JL_UMFPACK_DENSE_COL = UMFPACK_DENSE_COL + 1
+const JL_UMFPACK_PIVOT_TOLERANCE = UMFPACK_PIVOT_TOLERANCE + 1
 const JL_UMFPACK_BLOCK_SIZE = UMFPACK_BLOCK_SIZE + 1
 const JL_UMFPACK_ORDERING = UMFPACK_ORDERING + 1
 const JL_UMFPACK_FIXQ = UMFPACK_FIXQ + 1