New functions and optimization in time_series.py module

This MR provides the following additions to the time_series.py module:

• .pin_memory() for MultiModalSITS and MonoModalSITS
• .trim() for MonoModalSITS, which trims the doy dimension to the maximum number of valid dates over the batch
• sits_where() which performs a doy wise where operation between two MonoModalSITS
• Better handling of masks in concatenate_monomodal_sits() : masks are not required to be the same for all sits, and a pixel is valid in the output only if it is valid in all inputs
• Optimization of subset_doy_monomodal_sits() : do not perform memory hungry operations on the spatial dimensions since doy is the same for all pixels in a spatial patch.