Lua: Add "tonumber" method to NSTime

Lua: Add "tonumber" method to NSTime

Absolute and Relative time fields could not be converted to seconds
without converting to string and parsing to number.

Fixed conversion in generated code that was subject to precision loss

Usage:
f=Field.new("frame.delta_time")
delta=f().value:tonumber()

Change-Id: I6ef91c6238a6c2ed9adf6cae03f8913f0a09332e
Reviewed-on: https://code.wireshark.org/review/22316
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Guy Harris <guy@alum.mit.edu>