Rename framerate attribute to sample_frequency

[psomhorst]

In EIT-ALIVE/eitprocessing#290 we changed the EITData.framerate attribute to EITData.sample_frequency. This will go in effect probably in v.1.3.0.

For now, EITData.framerate will produce a DeprecationWarning.

Note that this issue should only be fixed after the dash will support eitprocessing>=v.1.3.0.

@wbaccinelli Is the dash fixed to a version of eitprocessing?

[wbaccinelli]

@psomhorst Yes, the eitprocessing version is fixed here. When upgrading the version to the new one, this issue has to be addressed.