Added warning related compilation flags and fixed related errors.

Hello, thanks for your great library !
I added some compilation flags and tested this with g++ under linux.
Several issues were encountered.
Hello David,

../include/fix8/field.hpp:943:10: error: missing initializer for member 'tm::tm_sec' [-Werror=missing-field-initializers]
  tm tms {};
          ^

Apparently, g++ is deliberately very severe with this : https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55805

I should have checked the standard in the first place ;) You're right, this is not relevant (and so are lots of the changes you commented later on).