Add prestress for fibers

Description

The paper by Hansbo et al. (2017) allows for a "load" \mathbf{f} to be applied on the 1D Euler-Bernoulli beams. This framework could be used to apply a prestress on the fibers. The load is decomposed into its tangential and normal components in the RHS,

l_\Sigma (v) = \left( R [f_t v_t, f_n v_n] \right)_\Sigma

which means that also a scalar quantity could be inserted. However, this looks structurally more like a body force.

However, this looks structurally more like a body-force. The prestress should enter as strech force and according to Hansbo et al. (2014) eqs. (52) and (72), it could look like this:

l_\Sigma(v) = EA \left( [\hat{\mathbf{t}} \otimes \hat{\mathbf{t}}] \sigma_\mathrm{pre}, [\hat{\mathbf{t}} \otimes \hat{\mathbf{t}}] \partial_t v \right)_\Sigma

In case of an isotropic prestress, a scalar value can be used: \sigma_\mathrm{pre} = \sigma \mathbf{I}. It is unclear if this formulation must then be projected onto the tangential or if this is not required.