Fix: field.hpp: various typos

Fix: field.hpp: various typos

 * `Common{Unsigned, Signed}IntegerField` is derived by the
   `Common{Unsigned,Signed}EnumerationField` class so can't be final.

 * `CommonSignedIntegerField::value` should be `int64_t`.

 * Calling single precision function instead of double.

 * `CommonStringField::operator=` methods can throw.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ib354f0bd4edc8f625494acedac1052c7f45ec67d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8185
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10828
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>