$DATA_LMT not really used (enough)

[teuben]

The dreampy3 toolkit, as well as various places in lmtoy, all use $DATA_LMT, if present. There is some code in lmtlst that hard codes the root path as '/data_lmt', which indeed is a symlink at the UMass machine. However, on personal laptops and desktops this may be hard to do if no admit privileges are given. The first default should thus be $DATA_LMT if present.

There is a lot of code in lmtslr that use a path= in one way or another, and a fair amount of change is needed to implement this.

[teuben]

all reader.reduce_*() readers now set path=None the default, resulting in the following order of resolution:

1. path given, use it
2. $DATA_LMT given, use it
3. use '/data_lmt', but this may fail on non LMT data reduction computers

[teuben]

Fixed two more cases in

    lmtslr/utils/ifproc_file_utils.py
    lmtslr/utils/roach_file_utils.py

where DATA_LMT was hardcoded.