0.3.0

Improvements:
B1986_nmax:
-The user can now input several add_arrays, and multiplications in add_equation are now partially supported.
-An optional argument COM has been added to force the degree-1 geoid terms to be zero (as required in a center-of-mass framework)
-Cleanup some synthax and updated error messages

Displacement_strain:
-SH_deriv_Store: Added option to input dtype. Using float32 instead of 64 can significantly reduce the array size for large lmax.
-SH_deriv_Store: Added the "grid" and lmaxgrid optional arguments (also to Displacement_strains). The grid can either be 'DH' or 'GLQ' following the convention of SHTOOLs. Grid shapes are updated accordingly GLQ: (lmaxgrid+1,2lmaxgrid+1), DH (2lmaxgrid+1, 2*(2*lmaxgrid+1)). The default grid is now GLQ, which should allow for faster computation of Y_lms derivatives and strains and reduces the file size of Y_lms.

Modified some of the examples in the jupyter notebook to demonstrate how one can use the add_equation options with multiplications and multiple add_arrays.

Corrections:
B1986_nmax:
-Correct a bug when using filtering with nmax != 1
-Fixed the degree-1 equation for flexure (based on Beuthe 2008)
Displacement_strain:
-Corrected Plt_tecto_Mars to include the last point in the Knapmeyer dataset.