scuba2: revised sky noise, add per-subarray dark noise thresholds

Dark noise values based on Jim Hoge's experience of what values
correspond to good setups.

Sky noise values taken from watching the stripchart for a couple of
nights.

We'll try these out for a while and see what happens.
scuba2: use the QA report for the weighted NEP

The effective NEP is no longer reported, though it is still written to
log files and index files.
SCUBA2: remove effective NEP from QA analysis

And insert a more realistic NEP spec based on the worst-performing
subarray.
SCUBA2: set verbose to false to avoid duplicate messages when deleting files
SCUBA2: do not print the effective NEP to the screen

Since almost no one understands how to interpret the effective NEP
means, it's not worth printing to the screen. However, it is still
recorded in log files.
SCUBA2: add location of SMURF config files to search path
SCUBA2: add NOISE_CALC and NOISE_FREQLO recipe parameters

NOISE_CALC can be quick or full and indicates how much data to use
when calculating the noise from on-sky data. Default is quick.

NOISE_FREQLO corresponds to the freqlo parameter for calcnoise.
SCUBA2: add support for NOISE_FREQLO recipe parameter
SCUBA2: define a suitable plotmax for NEP display

Plotmax wasn't being defined properly for displaying the NEP
image. Retrieve it from the NEP_SPEC uhdr, or simply determine the
peak value in the image.

Also be more verbose with output to user.
SCUBA2: add ability to be used by offline pipeline

Previously this primitive was only used by the QL pipeline to
determine the noise properties of science data. Now it can be used by
the offline pipeline (with a suitable recipe), and can either operate
on the first on-sky subscan or the entire timeseries.

Also fix a bug whereby the noise data weren't actually being used in
the QA by delaying the QA reporting for when the noise data are being
analyzed. This works because the weighted NEP determined in the first
call to _ANALYZE_NOISE_ is still stored in the QA report info hash.
SCUBA2: new recipe to allow offline noise analysis of science data

This recipe more-or-less repeats the analysis performed by the QL
pipeline, enabling observers to examine the noise properties of their
data and get a handle on the data quality.

The users can specify a config file to use in the noise calculation,
as well as the frequency range to be used. By default, the recipe will
only use the first 30 seconds of on-sky data, but the user can force
the entire observation to be used instead.