minor fixed / logger statements

[ssim]

Bugfixes
^^^^^^^^

• "print" replaced with logger for classical nebular
• logger statement added for coronal approximation
• warning added to documentation since plasma is out of date (temp
  solution only) No module named plasma / BasePlasmaArray AttributeError #108

[ssim]

@wkerzendorf
happy with these minor changes?

[wkerzendorf]

@ssim looks good - if travis passes just merge