Add pn.panel and ensure all Panes have layout attribute

[philippjfr]

@jbednar Please test this extensively.

• Implements #124
• Updates unit tests
• Update docs

[codecov-io]

Codecov Report

Merging #126 into master will decrease coverage by 0.24%.
The diff coverage is 94.05%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
- Coverage      90%   89.75%   -0.25%     
==========================================
  Files          27       27              
  Lines        4061     4023      -38     
==========================================
- Hits         3655     3611      -44     
- Misses        406      412       +6

Impacted Files	Coverage Δ
panel/tests/test_vega.py	96.96% <ø> (-0.05%)	⬇️
panel/tests/test_interact.py	100% <100%> (ø)	⬆️
panel/tests/test_layout.py	100% <100%> (ø)	⬆️
panel/plotly.py	100% <100%> (ø)	⬆️
panel/tests/test_param.py	99.61% <100%> (-0.01%)	⬇️
panel/__init__.py	62.5% <100%> (ø)	⬆️
panel/interact.py	71.7% <75%> (-5.84%)	⬇️
panel/param.py	90.27% <81.81%> (+1.04%)	⬆️
panel/holoviews.py	82.5% <88.88%> (-0.73%)	⬇️
panel/pane.py	81.94% <93.33%> (-0.12%)	⬇️
... and 3 more

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[jbednar]

Looks good. I'll test it.

[jbednar]

Should this not be a public class Parameter? Seems like something one could legitimately want to customize.

[philippjfr]

Always prefer if you warn me about making edits to a branch I'm actively working on. You can now also leave suggested edits directly in the GitHub, which let's me apply the changes when I'm ready.

[jbednar]

@jlstevens, consider code like this:

header = pn.Row(pn.create(oggm_logo, width=170), pn.Spacer(width=50), 
                pn.Column(pn.Pane(title, height=25, width=400), pn.Spacer(height=-15), pn.create(instruction, width=500)),
                pn.Spacer(width=180), pn.Column(pn.create(dyn_count), clear_button, pn.Spacer(height=-15)), 
                pn.create(pv_logo, width=80))

To me create reads oddly there; I'd rather see panel so that I can see that it's a row of rows, spacers, and panels, not a row of creates...

[jbednar]

the components?

[philippjfr]

@jbednar Any objections to merging this soon?