Move Solow to lectures location and add to _toc.yml

[mmcky]

This PR:

1. shifts solow lecture to new location
2. removes the _build folder

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[mmcky]

The solow lecture is missing color definition.

@jstac is this a matplotlib color object that we just need to import?

NameErrorESC[0m: name 'colors' is not defined

in

# simulate and plot time series
for x_init, col in zip(x0, colors):
    ts[0] = x_init
    for t in range(1, ts_length):
        ts[t] = g(ts[t-1])
    ax.plot(np.arange(ts_length), ts, '-o', ms=4, alpha=0.6, label=r'$k_0=%g$' %x_init)