More dot methods for grid classes

[gully]

Sujay and I had a good IRL chat about having more dot methods on the grid classes. The sibling package muler has these for the SpectrumCollection-like counterpart. Often these dot methods wrap the single-spectrum object with a simple for-loop over the spectrum list. So they're a bit unglamorous, and possibly slow. But they're convenient.

The specific example that came up was .instrumental_broaden( ), since we often have want to smooth the entire grid as soon as we read it in, when working with low res spectra for example.