Changes

## v0.7 Feb 28 2019

API modification:
- Breaking:
  - data parameter passed to physics callbacks only if Physics object is created with data parameter.
    This makes the API more consistent in the case that parameters are just taken from the closure (the scope where the physics functions are defined) and no data object has been created.
  - Replace node.coord[i] by node[i].
  - Replace edge.coordK[i] by edge[i,1].
  - Replace edge.coordL[i] by edge[i,2].
    This now directly accesses the coordinate field of the grid and avoids copying of the coordinates
- Backward compatible:
  - No need for viewK and viewL in edge callbacks (they also make trouble with allocations...)
    - Replace uk[i] by u[i,1]
    - Replace ul[i] by u[i,2]
  - Replace VoronoiFVM.DenseSystem(...) by VoronoiFVM.System(..., unknown_storage=:dense)
  - Replace VoronoiFVM.SparseSystem(...) by VoronoiFVM.System(..., unknown_storage=:sparse)
No allocations anymore in assembly loop:
- Replaced ElasticArray in Grid by normal one - this was the largest regression
- Return nothing from mutating methods to avoid some allocations
- Indexing in formfactors.jl with Int

remove xcolptrs call Update dependency on ExtendableSparse

Updated dependency list (Triangulate ^0.4.0)

(Temporary) Copy of TriangleRaw as Triangle submodule. To be replaced by dependency on evisioned package

triangle in optional submodule
Modified API for plotting
- Removed formal dependency on Plots and PyPlot
- Use Plotter module as first parameter to plot methods - replaces fvmplot and fvmpyplot functions. Use VoronoiFVM.plot(PyPlot,...) resp. VoronoiFVM.plot(Plots,...)
- No more complaints when package is used in environment with plots or pyplot installed
Modified API for impedance

Registered with Julia ecosystem
Enhance Newton solver by embedding, exception handling
Replace SparseMatrixCSC with ExtendableSparseMatrix
fixed allocation issues in assembly
assured that users get allocation stuff right via typed functions in physics structure
more julianic API

Renamed from TwoPointFluxFVM to VoronoiFVM
Complete rewrite of assembly allowing sparse or dense matrix to store degree of freedom information
- Solution is a nnodes x nspecies sparse or dense matrix
- The wonderful array interface of Julia still provides slicing etc in oder to access species without need to write any bulk_solution stuff or whatever when using the sparse variant
Re-export value() for debugging in physics functions
Test function handling for flux calculation
First working steps to impedance handling
Abolished Graph in favor of Grid, Graph was premature optimization...

Changed signature of all callback functions: This also allows to pass user defined arrays etc. to the callback functions. In particular, velocity vectors can be passed this way.
- Besides of flux!(), they now all have node::VoronoiFVM.Node as a second argument.
- flux!() has edge::VoronoiFVM.Edge as a second argument
- the x argument in source!() is omitted, the same data are now found in node.coord