fix cubes: make output headers conform to FITS standard when cropping
galactic images
formaldehyde_mm: more flexible file reading
read_class: skip reading the data when data reading is to be skipped
dramatically speed up class reader by simply not being dumb
allow LMFIT tied parameters and let cubes be headerless
have to make a stupid assumption if no header is given
zomg... has to be a Header() I guess
in fitting, I did some things with headers that required the existence
of headers
allow xarr to be initialized from an astropy Quantity
units -> unit
update changelog
added new example initializing a spectrum from numpy array
add codeblock
added a class_v2 reader, but it's still under development and doesn't
work
cleanup of baseline - don't allow mismatched masks
memmaping in read_class and enhancements for v2 - almost there!  can
read scan-by-scan now
v2 reader is fully functional; next commit replaces v1 with v2
read_class now just uses the object
ok I lied there are a series of bugfixes that needed to be done first
replace old version with new version.  mmap ftw
add a debug line: can't have empty headers
add support for continuum chunks