Change default value of depth in equivalent sources

[santisoler]

Change the default value of the depth argument in EquivalentSources to "default". If this value is passed, then the depth of the sources is set as 4.5 times the median distance between first neighbouring sources. If a numerical value is passed, this is the one that will be used. Introduce a new depth_ attribute where the estimated/passed numeric value is stored. If points is passed, the depth_ attribute is set to None. Add a test checking that the depth value is properly set. Merge the tests functions that checked the accuracy of equivalent sources using different dtypes.

Relevant issues/PRs:

Fixes #424