ENH: use yaml config files to define models

[kaczmarj]

the wsinfer run command now accepts a --config option to a yaml file. the options --model and --weights are mutually exclusive with --config.

this is an example of a yaml config file:

# Configuration of a breast cancer tumor detection model.
# The models are referenced by the pair of [architecture, weights], so this pair must
# be unique.
architecture: resnet34  # Must be a string.
name: TCGA-BRCA-v1  # Must be a string.
# Where to get the model weights. Either a URL or path to a file.
# If using a URL, set the url_file_name (the name of the file when it is downloaded).
url: https://stonybrookmedicine.box.com/shared/static/dv5bxk6d15uhmcegs9lz6q70yrmwx96p.pt
url_file_name: resnet34-brca-20190613-01eaf604.pt
# If using a relative path, the path is relative to the location of the yaml file.
# file: /path/to/weights.pt
num_classes: 2
transform:
  # These are keyword arguments to the PatchClassification class.
  resize_size: 224  # Must be a single integer.
  mean: [0.7238, 0.5716, 0.6779]  # Must be a list of three floats.
  std: [0.1120, 0.1459, 0.1089]  # Must be a list of three floats.
patch_size_pixels: 350
spacing_um_px: 0.25
class_names:
  - notumor
  - tumor