Make copy after slicing ShiftConv Doppler bank results instead of before so that the
result is C-contiguous.
Cast to smallest possible int type when parsing Jicamarca headers so unnecessary longs
don't get propagated.
Add check for header validity (time > 0) when mapping blocks in a Jicamarca file. This
prevents us from trying to read a block that doesn't exist in the file.

Also add a function for recursively finding Jicamarca data files in a folder.