fixed Date throwing an exception when its value > LONG_LONG_MAX

This fixes the following exception when attempting to print a date:

```
too large for defined data type
-9223372036854775808
9.22337e+18
----------------- Exception thrown ------------------------
type: ML::Exception
pid: 26285; tid: 22586
what: problem with gmtime_r
stack:
00: 0x0x7f8bcde85e17 at __cxa_throw + 0x27 in build/x86_64/bin/libexception_hook.so + 0xe17build/x86_6
4/bin/libexception_hook.so(__cxa_throw+0x27) 0x7f8bcde85e17
01: 0x0x7f8bcbe2d8e7 at Datacratic::Date::print(std::string const&) const + 0x207 in build/x86_64/bin/
libtypes.so + 0x1f8e7build/x86_64/bin/libtypes.so(_ZNK10Datacratic4Date5printERKSs+0x207) [0x7f8bcbe2d
8e7]
02: 0x0x7f8bcbe2dd2c at Datacratic::Date::print(unsigned int) const + 0xac in build/x86_64/bin/libtype
s.so + 0x1fd2cbuild/x86_64/bin/libtypes.so(_ZNK10Datacratic4Date5printEj+0xac) 0x7f8bcbe2dd2c
03: 0x0x7f8bcbe2de27 at Datacratic::operator<<(std::ostream&, Datacratic::Date const&) + 0x17 in build
/x86_64/bin/libtypes.so + 0x1fe27build/x86_64/bin/libtypes.so(_ZN10DatacraticlsERSoRKNS_4DateE+0x17) [
0x7f8bcbe2de27]
```

Note that there is some loss in precision, in that dates that can't fit in 64 bits of seconds will come out as "Inf" when printed.