Small improvements for the discrete PT1 and PT2 blocks in 'filters_lib'

[jwithelm]

All of the following suggestions would be breaking changes, meaning that any library or model using these blocks would need to be revised. The proposed changes are not important and could be made if another important breaking change were made.

• Rename block: 'PT1 discrete with saturations' -> 'PT1 discrete with saturation'
• Rename mask parameter names for discrete PT1 and PT2:
  • initial output parameter names are not consistent, they are e. g. y_0_src and x_init. It would probably be better to name them y_0_src and y_0.
  • the naming of output min/max parameters and initial output parameters should also be consistent?
• Change the PT2 block, so that omega and d can be set separately to 'internal / external' .