Make matplotlib imports optional

[pfs@1cfe7a2b3699 ics_drpActor]$ export TARGET=/drp/lam
[pfs@1cfe7a2b3699 ics_drpActor]$  export RERUN=pfs
[pfs@1cfe7a2b3699 ics_drpActor]$ export batchArgs="--batch-type=none --doraise"
[pfs@1cfe7a2b3699 ics_drpActor]$ constructFiberFlat.py $TARGET --calib $TARGET/CALIB --rerun $RERUN/calib --id visit=9627..9692 $batchArgs
Traceback (most recent call last):
  File "/opt/lsst/software/stack/stack/miniconda3-4.3.21-10a4fa6/Linux64/drp_stella/1d712c3c7fd8146b89ce5b803da2e0ac5ef99ad7/bin/constructFiberFlat.py", line 2, in <module>
    from pfs.drp.stella.constructFiberFlatTask import ConstructFiberFlatTask
  File "/opt/lsst/software/stack/stack/miniconda3-4.3.21-10a4fa6/Linux64/drp_stella/1d712c3c7fd8146b89ce5b803da2e0ac5ef99ad7/python/pfs/drp/stella/__init__.py", line 3, in <module>
    from .spectraContinued import *
  File "/opt/lsst/software/stack/stack/miniconda3-4.3.21-10a4fa6/Linux64/drp_stella/1d712c3c7fd8146b89ce5b803da2e0ac5ef99ad7/python/pfs/drp/stella/spectraContinued.py", line 12, in <module>
    from pfs.datamodel.pfsArm import PfsArm, PfsConfig
  File "/opt/lsst/software/stack/stack/miniconda3-4.3.21-10a4fa6/Linux64/datamodel/264fb7b46bfa4c56f9c31e5b128850ba3f4a4776/python/pfs/datamodel/pfsArm.py", line 13, in <module>
    import matplotlib.pyplot as plt
  File "/software/conda/lib/python3.6/site-packages/matplotlib/pyplot.py", line 31, in <module>
    import matplotlib.colorbar
  File "/software/conda/lib/python3.6/site-packages/matplotlib/colorbar.py", line 36, in <module>
    import matplotlib.contour as contour
  File "/software/conda/lib/python3.6/site-packages/matplotlib/contour.py", line 20, in <module>
    import matplotlib.font_manager as font_manager
  File "/software/conda/lib/python3.6/site-packages/matplotlib/font_manager.py", line 55, in <module>
    from matplotlib import afm, cbook, ft2font, rcParams, get_cachedir
ImportError: /opt/lsst/software/stack/python/miniconda3-4.3.21/lib/python3.6/lib-dynload/../../libz.so.1: version `ZLIB_1.2.9' not found (required by /software/conda/lib/python3.6/site-packages/matplotlib/../../.././libpng16.so.16)