standardize output fields for property types

[chrishavlin]

The docs should include a description of the standard output fields for elastic, viscous and anelastic methods.

[chrishavlin]

for anelastic methods:

frequency dependent variables:

• J1, J2 : frequency dependent storage and loss modulus in 1/Pa
• Q : quality factor
• Qinv: attenutation (1/Q)
• M: (shear) modulus in Pa
• V: shear wave velocity in m/s

frequency independent variables

• tau_M: maxwell time in s
• Vave: average shear wave velocity across frequencies

other expected variables

• units: a structure containing the units of the output field.

Note: for units for anelastic methods, can populate standard units entries with Q_method_units(). any non-standard fields should have corresponding units entries.

[chrishavlin]

Note: the individual methods pages linked from https://vbr-calc.github.io/vbr/vbrmethods/anelastic/ do include output field descriptions. but it'd be nice if https://vbr-calc.github.io/vbr/vbrmethods/anelastic/ itself had a list of the output fields that are common to all methods.