ENH: TraceFileConvert Can Export Relative Times

[zdomke]

Description

Before this PR, if a user enabled autoscrolling and then saved the file, the x-range timespan was only saved as absolute times. This is not what the user would expect and would be unhelpful when loading a file.

This PR will allow TraceFileConvert to export the timespan as a relative time. This relative time will be in the right format to enable autoscroll when a user imports the given file.

Motivation

Closes #122
Closes Jira ticket: SWAPPS-50

Example output:

"time_axis": {
"name": "Main Time Axis",
"start": "-1M -10H -5m",
"end": "now",
"location": "bottom"
}

[zdomke]

Ran into a problem caused by having trace_file_convert.py import another Trace file. I removed this import and moved all functionality back into the TraceFileConverter class.

I'd prefer to to import IOTimeParser and make use of IOTimeParser.time_re. I'd also prefer that TraceConverterFile.delta_to_relative was actually a method in IOTimeParser. Maybe these changes can be implemented if Trace's imports get fixed.

[YektaY]

looks good to me