Sort and document containers

[m-weigand]

At this point it is not clear which container is used for what.

At the moment we have the following containers:

• ERT: electrical resistivity data (magnitude)
• sEIT: spectral electrical impedance data (spectral, complex, imaging)
• SIP: spectral induced polarization data (spectral, complex)

complex=magnitude+phase|real+imag

I propose to add:

• TDIP: (magnitude + time-domain decay data in the form of chargeabilities or decay curves)
• CR: complex resistivity (single-frequency, complex, imaging)

Note 1: Perhaps we should design the ERT, TDIP, CR classes to that they inherit from each other in the order ERT -> TDIP -> CR. Thus, CR would contain all functionality contained on TDIP and ERT (however, certain functionality would be optional only in the case that chargeabilities are present).

In Theory this would also extend to the sEIT container, e.g. in the case were spectral data is recovered from TD-IP measurements. However, this would most probably be realized by means of some SIP-model such as the Cole-Cole model, and perhaps a new container would be appropriate once we want to support this.

Note 2: the TDIP and ERT would be closely related, and CR and sEIT. It would be a good idea to provide simple conversion functions.

Note 3: TDIP (i.e. chargeabilities) can be converted to CR (phase values) by using some approximations and analytical assumptions, which boils down to multiplying the integral chargeability by a factor (something like 1.5). However, I strongly suggest to make this conversion explicit (i.e. by the use of different container names and specific conversion functions).