Support for interpolation

[richfitz]

Uses https://github.com/mrc-ide/dust/pull/415 so merge that first and update the branch pin.

This PR allows for basic interpolation, subject to some conditions:

• ~only piecewise constant and linear is supported~
• the interpolated y must be a scalar (i.e. no multivariable targets supported yet)
• does not work in discrete time models

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Files	Coverage Δ
R/generate_dust.R	100.00% <100.00%> (ø)
R/generate_dust_equation.R	100.00% <100.00%> (ø)
R/utils.R	100.00% <100.00%> (ø)

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