Added missing pose modality

[JohannesK14]

As mentioned in #73 the dataloader fails if the "pose" modality is requested. This change simply adds it to the list of modalities in

tartanairpy/tartanair/tartanair_module.py

Line 51 in fd71087

self.modality_names = ['image', 'depth', 'seg', 'imu', 'lidar', 'flow', 'event']

which results in the pose being loaded alongside with the images, depths, etc.