Stochastic Gradient Descent (SGD) example and functionality

[iancze]

This feature branch includes changes to introduce Stochastic Gradient Descent workflow

• Reorganize SimpleNet and the entire GriddedDataset workflow as an alternative option to SGD, with documentation about the use cases when one or the other might be preferred. closes New quickstart and examples #251
• Documentation/"Getting Started" to explain how MPoL library usage follows the the normal ML paradigm with SGD closes Framing model.forward more like ML libraries? #188
• SGD example shows how to create a TensorDataset and a DataLoader closes Redesign UVDataset with Pytorch idioms in mind #162
• Added a fixed FWHM Gaussian convolution routine to set base resolution.