Allow user set data reading lengths in pulsar heterodyne code
Currently the maximum length of data read in from a frame file in lalapps_heterodyne_pulsar
is hardcoded to be 256 seconds. This patch instead allows this to instead be set by the user, which can results in faster run times if set to a larger number. It also allows this parameter to be set for use by the lalapps_knope
pipeline.
cc - @karl-wette