Adding a new VBR core method

To add a new method to the VBR core:

Open the corresponding parameter file in vbr/vbrCore/params and then:
- add the new method name to the params.possible_methods cell array
- add an elseif catch for the new method name
- within the elseif, set param.func_name to the name of the matlab function that you will write for the new method, e.g.,
```
  param.func_name='new_vbr_method'
```
- set any other values/parameters that the new method needs.
Create a new file in vbr/vbrCore/functions for your new function with the name from param.func_name. Using the above example, that would be new_vbr_method.m.
Write your new method function. The function must have the VBR struct as input and output:
```
function [VBR] = new_vbr_method(VBR)
```
The remainder of the function is where you write whatever calculations are appropriate. The VBR structure will come in with all the state variables and parameter values. State variables are accessed with, e.g., VBR.in.SV.T or VBR.in.SV.phi. The parameter values are accessed with VBR.in.method_type.method_name where method_type is the same as the parameter file that you modified (anelastic,elastic or viscous) and method_name is the name you added to params.possible_methods.
To return the results of your function, modify the VBR.out structure appropriately, e.g., VBR.out.method_type.method_name.result = result; where method_type is the same as the parameter file that you modified (anelastic,elastic or viscous) and method_name is the name you added to params.possible_methods
If your new method relies on other methods (e.g., you’re putting in a new anelastic method that requires an elastic method to exist), you can add your method to vbr/vbrCore/functions/checkInput.m following the other methods already there.

To use your new method, simply add the new method name to the methods_list, before you call VBRspine, e.g.:

VBR.in.method_type.methods_list={'method_name'}

where method_type is anelastic,elastic or viscous and method_name is your new method.