Resolve "Updating xmin and xmax"

When Σ- is executed, it checks:

• if any element in the last row, equals xmax or ymax. If it it does, then it rescans the matrix's remaining rows for max. Same procedure for min. If none are equal, max and min do not change after the last row is deleted and therefore stays unchanged.

• it also subtracts all the last row elements from all the accumulators.

• If any of the values above are found to be invalid, infinity or so, there is an issue, and the sums will reset and the matrix re-scanned for all accumulators and max/min.

There was a bug which I fixed. The example below is of the merge request.

Example:

1 ENTER Σ+ 2 ENTER Σ+ 3 ENTER Σ+ 1.5 ENTER Σ+
RCL STATS
[ 1 1 ]
[ 2 2 ]
[ 3 3 ]
[ 1.5 1.5 ]
XMAX
ymax = 3
xmax = 3
Σ-
RCL STATS
[ 1 1 ]
[ 2 2 ]
[ 3 3 ]
XMAX
ymax = 3
xmax = 3
Σ-
RCL STATS
[ 1 1 ]
[ 2 2 ]
XMAX
ymax = 2
xmax = 2

Closes #985 (closed)