Fix 'the_* landau notation to improve readability

[CohenCyril]

• Make the notation f = g +o_e stable (e.g. by putting a definition)
  

  analysis/landau.v

  Line 199 in 1566e32

  Notation "f = g '+o_' F h" :=

• Remove the "'the" tag which makes things unreadable
  

  analysis/landau.v

  Line 135 in 1566e32

  Definition the_tag : unit := tt.

  
  ...

[CohenCyril]

Half fixed in commit 9c7a6dd, the tag the_tag remains but is printed 'o_x e.