TransientGridSearch: new F_mn writer function matching the format of lalpulsar.write_transientFstatMap_to_fp

 -optional import of pyCUDA package only if requested
 -including kernel .cu files in packaging

 -to later allow for CUDA implementation
 -some optional-import acrobatics
  to fail gracefully if a module (especially CUDA)
  is not available
 -as of this commit, nothing beyond lal implemented yet

writer function for atoms (TransientGridSearch)

See merge request GregAshton/PyFstat!14

Closes #12

displayless matplotlib import workaround also in helper_functions.py

See merge request GregAshton/PyFstat!12

transients: fix window=none case and allow custom dt0, dtau

See merge request GregAshton/PyFstat!11

 -first search should be standard non-transient GridSearch for comparison

Create TransientGridSearch to provide transient grid searches

Closes #11

See merge request GregAshton/PyFstat!10

Previously, this functionality was part of GridSearch, this splits this
into a separate subclass to ease future development.

Previously this returned True regardless of the value of
transientWindowType when really we want to also check if it is not None

- Default to 1 glitch
- Only try to calculate segment 2F if length > 1800s

add transient F(t0,tau) file output

See merge request GregAshton/PyFstat!8

 -"AttributeError: 'function' object has no attribute 'gridcorner'"

 -new output: full (t0,tau) Fstat map,
  if bands in those params are searched
 -one file per Doppler grid point

Closes #7