Allow `ReadStrain` object to be re-used for different strain time series

When a ReadStrain object is created,

• Metadata and internal variables are set
• The internal time vector is computed and stored
• Spacecraft positions and LTTs are interpolated and interpolants are stored (can be bypassed using arguments of ResponseFromStrain, as we do for stochastic backgrounds)
• Source frame basis vectors (k, u, v) are computed
• Strain h_+, h_\times are interpolated and interpolants are stored

Some studies necessitate to quickly compute the response for different strain time series, but keeping other parameters fixed. This is the case for stochastic sampling of likelihood surfaces using templated generated by this simulator. Therefore, creating a new ReadStrain object is suboptimal and we should offer a way to update the strain time series on the fly.

A possible solution is to define a custom setter for hplus and hcross , such that they are interpolated when set.