Method with Digital Signal Processing for Increasing the Efficiency of Thermal Neutron Detection by a Scintillation Detector Based on a $${{^{6}{\text{LiF}}} \mathord{\left/ {\vphantom {{^{6}{\text{LiF}}} {{\text{ZnS}}({\text{Ag}})}}} \right. \kern-0em} {{\text{ZnS}}({\text{Ag}})}}$$ Screen and Wavelength-Shifting Fibers