Use Time variable instead of TimeInterval attribute

Previously ncplot has always used the TimeInterval global attribute to determine start and end time for the file. This was a fairly RAF specific attribute from 1992/1994. Switch to using the Time variable, which will be more generic and allow more user created files to just work.

Add missing point count on plot

A new option has been added, on the control window at bottom right 'Missing value count'. This toggle will display a count of # of consecutive missing points on the plot. Helps identify data gaps and the magnitude.

Fix autoscale button for XY, XYZ, and diff plots

Autoscale and autotics buttons have multiple callbacks. They are sensitive to the order they are called. XY, XYZ and diff plots had them backwards.

Repair wind barbs causing core dump.

Fixes an error introduced in 2017. Wind vectors were causing a core dump.

Improve box-zoom behavior at right edge

When going past the right edge, we were still rounding to the nearest minute. Now keep the original right boundary. This is frequently associated with looking at the end of a flight, the previous behavior was rounding back a minute providing a false sense of the end of flight.

Refactor Help Menu

Previously the help menu forked firefox directly to resolve the URLs. Now use the OS utilities (open or xdg-open) to open a tab in the default browser.

Maintenance release

Changelog:

ASCII Window:

• increased maximum number of lines from 1000 to 5000.
• round non-integer sample rates up to nearest whole number.

Adjust some window widths.

Makefile: remove obsolete flags.