`lstsq`: return correct array size

[perazz]

Fix #817.

for Ax=b with shape(A) == [m, n]:

• Subroutine interface: allow size(x,1)>=n.
  • if also size(x,1)>=m: use as temporary storage and do not reallocate
  • otherwise: allocate temporary for *GESV interface
• Function interface: always return size(x,1)==n

cc: @jvdp1 @jalvesz @loiseaujc

[perazz]

Thanks a lot @jvdp1. I will wait another couple of days and then merge, if there are no further comments.