allow sorting and printing of MesaData objects

[zingale]

These changes allow one to print and sort (by age) MESA profiles. E.g.,

profiles = glob.glob("profile*0.data")
models = []
for p in profiles:
    models.append(mr.MesaData(p)

for m in sorted(models):
    print(m)

produces

MESA model #    450, t =          158677.1499 yr
MESA model #    900, t =          9371689.143 yr
MESA model #   1350, t =          934449849.7 yr
MESA model #   1850, t =          994276962.1 yr
MESA model #   2350, t =           1011636738 yr
MESA model #   2850, t =           1021325384 yr
MESA model #   3350, t =           1027557774 yr
MESA model #   3850, t =           1031542396 yr
MESA model #   4350, t =           1034313116 yr
MESA model #   4850, t =           1036342629 yr
MESA model #   5350, t =           1037887011 yr
MESA model #   5850, t =           1039104992 yr
MESA model #   6350, t =           1040087688 yr
MESA model #   6850, t =           1040898244 yr
MESA model #   7350, t =           1041578008 yr
MESA model #   7850, t =           1042156309 yr
MESA model #   8350, t =           1042653626 yr
MESA model #   8850, t =           1043085731 yr
MESA model #   9350, t =           1043465657 yr
MESA model #   9850, t =           1043801929 yr

[wmwolf]

Well this somehow went a year and a half without my remembering, but it seems like a welcome addition. Thanks, Mike!