Refactor s/M/EEG monitors

Following discussions, the s/M/EEG monitors shall be refactored as follows:

• A base class "LeadFieldMonitor" is to be implemented which provides the basic functionality of temporal averaging and applying a lead field (a matrix) to the raw activity, obtaining sensor activity

• Subclasses for sEEG, EEG, and MEG will provide modality-specific analytic solutions if no lead field matrix is present

The so-called Spherical classes will no longer exist, and in the UI, a user will simply choose sEEG, EEG, MEG as they wish, optionally choose a lead field matrix or be happy with the spherical approximations.