x2sys_cross: Refactor to get rid of temporary files and have consistent table-like output behavior

[seisman]

Originally posted by @weiji14 in #2730 (comment)

I had a look at refactoring x2sys_cross to use virtualfiles instead of temporary files, but it's a little tricky because:

• Input: Cannot pass in virtualfiles as input as mentioned at Wrap x2sys_init and x2sys_cross #546 (comment) and Passing in virtual files into the supplementary x2sys modules gmt#3717, since GMT doesn't support virtualfiles to X2SYS modules

• Output: The virtualfile_to_dataset method from clib: Add virtualfile_to_dataset method for converting virtualfile to a dataset #3083 was able to produce a pandas.DataFrame output, but the column names were missing. The logic for handling x2sys_cross's output is actually complicated:

  pygmt/pygmt/src/x2sys_cross.py

  Lines 231 to 250 in bcbbcad

  	# Read temporary csv output to a pandas table
  	if outfile == tmpfile.name: # if outfile isn't set, return pd.DataFrame
  	# Read the tab-separated ASCII table
  	date_format_kwarg = (
  	{"date_format": "ISO8601"}
  	if Version(pd.__version__) >= Version("2.0.0")
  	else {}
  	)
  	table = pd.read_csv(
  	tmpfile.name,
  	sep="\t",
  	header=2, # Column names are on 2nd row
  	comment=">", # Skip the 3rd row with a ">"
  	parse_dates=[2, 3], # Datetimes on 3rd and 4th column
  	**date_format_kwarg, # Parse dates in ISO8601 format on pandas>=2
  	)
  	# Remove the "# " from "# x" in the first column
  	table = table.rename(columns={table.columns[0]: table.columns[0][2:]})
  	elif outfile != tmpfile.name: # if outfile is set, output in outfile only
  	table = None

Important things to handle are:

1. Datetime columns need to be parsed correctly as datetime64 dtype
2. x2sys_cross may output multi-segment parts (see https://docs.generic-mapping-tools.org/6.5/supplements/x2sys/x2sys_cross.html#remarks) when multiple tracks are passed in and -Qe (external COEs) is selected. Unsure how this is handled in GMT virtualfiles (note that we actually just merge all the multi-segments into one table when pandas.DataFrame output is selected, output to file will preserve the segments though).
3. Last two column names can either be z_X/z_M or z_1/z_2 depending on whether trackvalues/-Z argument is set.

It should be possible to handle 1 and 3 somehow, but I'm not so sure about 2 since it will involve checking how GMT outputs virtualfiles in x2sys_cross. We'll need to do some careful checking to ensure the refactoring doesn't modify the output and makes it incorrect.