Interpolate local activation times

[p-j-smith]

Changes made:

• Add a helper function to interpolate local activation times onto the surface of a mesh, openep.case.case_routines.interpolate_activation_time_onto_surface
• Defaults to using a RBF interpolator with the same settings as openep.case.case_routines.interpolate_voltage_onto_surface

[codecov-commenter]

Codecov Report

Merging #162 (e03cf7e) into dev (728f50b) will decrease coverage by 1.05%. The diff coverage is 7.69%.

@@            Coverage Diff             @@
##              dev     #162      +/-   ##
==========================================
- Coverage   69.65%   68.59%   -1.06%     
==========================================
  Files          11       11              
  Lines         748      761      +13     
  Branches       96       96              
==========================================
+ Hits          521      522       +1     
- Misses        217      229      +12     
  Partials       10       10              

Impacted Files	Coverage Δ
openep/case/case_routines.py	79.62% <7.69%> (-6.28%)	:arrow_down:

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