fix bug pdb_selaltloc.py

[ShintaroMinami]

In function select_altloc(), residue numbers were compared as string. This was causing unexpected behavior in occasion. For example, in this test case, residue 9 was removed in output.

ATOM     31  CA  PRO A   5     330.251  77.884  99.983  1.00  9.79           C
ATOM     39  CA ASER A   6     329.416  79.995 103.003  0.11 12.15           C
ATOM     40  CA BSER A   6     329.428  80.056 102.954  0.89 12.06           C
ATOM     50  CA  SER A   7     333.035  80.324 104.142  1.00 12.28           C
ATOM     56  CA  VAL A   8     336.686  79.643 103.388  1.00 12.51           C
ATOM     64  CA ASER A   9     339.751  80.077 105.586  0.25 14.04           C
ATOM     65  CA BSER A   9     339.751  80.141 105.546  0.75 13.93           C
ATOM     75  CA  ALA A  10     343.449  80.372 104.774  1.00 14.02           C

The "resnum" and "prev_resnum" got casted to numeric in this pull request.