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 #251
• Documentation/"Getting Started" to explain how MPoL library usage follows the the normal ML paradigm with SGD closes #188
• SGD example shows how to create a TensorDataset and a DataLoader closes #162
• Added a fixed FWHM Gaussian convolution routine to set base resolution.