empymod/empymod: empymod v1.6.1

Add the possibility to calculate secondary fields only (excluding the direct field) by passing the argument xdirect=None. The complete xdirect-signature is now (only affects calculation if src and rec are in the same layer):

• If True, direct field is calculated analytically in the frequency domain.
• If False, direct field is calculated in the wavenumber domain.
• If None, direct field is excluded from the calculation, and only reflected fields are returned (secondary field).

Bugfix in model.analytical for ab=[36, 63] (zeroes) [#16].