Clear up deprication warning

[waltsims]

WARNING:root:DeprecationWarning: Attributes will soon be typed when saved and not saved

#229 (comment)

The offending line of code can be found here.

[waltsims]

          #289 is not compatible with binary execution:

┌───────────────────────────────────────────────────────────────┐
│                  kspaceFirstOrder-CUDA v1.3                   │
├───────────────────────────────────────────────────────────────┤
│ Reading simulation configuration:                      Failed │
└───────────────────────────────────────────────────────────────┘
┌───────────────────────────────────────────────────────────────┐
│            !!! K-Wave experienced a fatal error !!!           │
├───────────────────────────────────────────────────────────────┤
│ Error: Cannot read from attribute file_type in dataset [/.]:    │
│        iostream error                                         │
├───────────────────────────────────────────────────────────────┤
│                      Execution terminated                     │
└───────────────────────────────────────────────────────────────┘

reverting until this is fixed

Originally posted by @waltsims in #292 (comment)