Add interpolatingfunctions example to docs build

[fgasdia]

The example on using interpolating function for the ionosphere profiles was not built with the rest of the docs.

• The example uses a private repository because of license restrictions.
• The timing results for the different interpolators at the end is also surprising and should be checked

[fgasdia]

Interp.jl should be replaced with a monotonic interpolator from Interpolations.jl before merging.

[fgasdia]

The docs now use an FIRI profile instead of Wait and have replaced Interp with three monotonic interpolators from Interpolations.jl

[fgasdia]

Corresponds to #43