Figure.legend: Refactor to simplify the logic of checking legend specification

[seisman]

Description of proposed changes

This PR contains a subset of changes in PR #3326. I open this PR so that PR #3326 can be smaller and easier for review.

This PR refactors the following codes:

withSession() aslib:
ifspecisNone:
specfile=""elifdata_kind(spec) =="file"andnotis_nonstr_iter(spec):
# Is a file but not a list of filesspecfile=specelse:
raiseGMTInvalidInput(f"Unrecognized data type: {type(spec)}")
lib.call_module(module="legend", args=build_arg_list(kwargs, infile=specfile))

to

kind=data_kind(spec)
ifkindnotin {"vectors", "file"}: # kind="vectors" means spec is NoneraiseGMTInvalidInput(f"Unrecognized data type: {type(spec)}")
ifkind=="file"andis_nonstr_iter(spec):
raiseGMTInvalidInput("Only one legend specification file is allowed.")
withSession() aslib:
lib.call_module(module="legend", args=build_arg_list(kwargs, infile=spec))

The pros are:

• Raise exceptions if any before entering a session
• Better error message if multiple legend files are given

[seisman]

This line can also be written as:

if spec is not None or kind != "file"

It's written in the current way so that this line can be extended to check "stringio" kind with minor changes (xref: #3326).

It's not straightforward to see why kind="vectors" means spec is None. That's also why I want to refactor the data_kind function in #3351.