Taup fix for models with no discontinuities

[johnrudge]

What does this PR do?

This PR fixes the issue described by @dylanmikesell and @kaspervanwijk in the comments of #3070, namely the failure to build a tau model when there are no discontinuities present. With this PR, the one layer model example now works:

from obspy.taup import TauPyModel
from obspy.taup.taup_create import build_taup_model
build_taup_model('1_layer_model.tvel')
model = TauPyModel("1_layer_model")
arrivals = model.get_ray_paths(distance_in_degree=160.0, source_depth_in_km=0.0)
print(arrivals)

Building obspy.taup model for '1_layer_model.tvel' ...
filename = 1_layer_model.tvel
Done reading velocity model.
Radius of model  is 6371.0
Using parameters provided in TauP_config.ini (or defaults if not) to call SlownessModel...
Parameters are:
taup.create.min_delta_p = 0.1 sec / radian
taup.create.max_delta_p = 11.0 sec / radian
taup.create.max_depth_interval = 115.0 kilometers
taup.create.max_range_interval = 0.04363323129985824 degrees
taup.create.max_interp_error = 0.05 seconds
taup.create.allow_inner_core_s = True
Slow model  224 P layers,382 S layers
Done calculating Tau branches.
Done Saving /home/john/Documents/students/stuart/obspy/obspy/taup/data/1_layer_model.npz
Method run is done, but not necessarily successful.
10 arrivals
	P phase arrival at 2023.939 seconds
	PP phase arrival at 2642.064 seconds
	PP phase arrival at 3148.690 seconds
	S phase arrival at 3514.964 seconds
	PS phase arrival at 3796.746 seconds
	SP phase arrival at 3796.746 seconds
	PS phase arrival at 4417.046 seconds
	SP phase arrival at 4417.046 seconds
	SS phase arrival at 4588.459 seconds
	SS phase arrival at 5468.313 seconds

These arrivals agree with the java version.

Why was it initiated? Any relevant Issues?

See #3070.

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[johnrudge]

@megies, I dropped one extra change into this PR because I noticed that taup_create is broken with the new pathlib changes you made (there's a print statement that expects a string as self.filename).

[megies]

rebased and force pushed. thanks @johnrudge