implement `curl`

fixes #7
implement `divergence`

fixes #9
implement `grad`
implement `Laplacian`

fixes #24
apply csymbol converts name to lower case

<apply>
  <csymbol>name</csymbol>
  .
  .
  .

produces the same output as

<apply>
  <name>
  .
  .
  .

Except tag names are automatically put in lower case, so csymbol names should be too.
divergence and curl decide when to bracket their arg