Allow list hyperparameters in method JSON

[bcyphers]

First, this PR moves a lot of hyperparameter logic from BTB to ATM. Specifically, all conditional hyperparameters are handled on the ATM side now. Categorical hyperparameters will remain in BTB, but the BTB categoricals will not be relevant to ATM anymore.

The bigger change is the addition of List hyperparameters: you can now add a single hyperparameter that represents a list of many hyperparameters of the same type and range. For example, in multi_layer_perceptron.json:

        "hidden_layer_sizes": {
            "type": "list",
            "sizes": [1, 2, 3],
            "element": {
                "type": "int",
                "range": [2, 300]
            }
        },

Lists need to be defined with caution. Behind the scenes, the list is translated to a single "size" hyperparameter along with a list of "element" hyperparameters conditioned on every possible value of "size". For example, the above code would define the following hyperpartitions:

hidden_layer_sizes_size = 1
  hidden_layer_sizes[0] = ?

hidden_layer_sizes_size = 2
  hidden_layer_sizes[0] = ?
  hidden_layer_sizes[1] = ?

hidden_layer_sizes_size = 3
  hidden_layer_sizes[0] = ?
  hidden_layer_sizes[1] = ?
  hidden_layer_sizes[2] = ?

Note that "sizes" must be defined as a list of possible values, not a range. This should make it harder to accidentally create O(n²) conditional hyperparameters by defining a list with a huge range.

Finally, categorical hyperparameters are now defined with a list of "values" instead of a "range". This is meant to make it less confusing when defining int or int_cat parameters. For example:

"param_a": {
    "type": "int_cat",
    "values": [1, 3]         # Only includes the values 1 and 3
},

is different from

"param_b": {
    "type": "int",
    "range": [1, 3]         # Includes the values 1, 2, and 3
},

[bcyphers]

Updated: I'm including custom method passing and absolute paths with this PR for convenience.

From the commit:
"add custom method definitions: users can pass a path to method JSON rather than a method code
in addition, all paths should now be relative to the project root (rather than ./)"

This now addresses #61 and part of #52.